Add "spot the error" sections for Stage 1B

examples/stage1/stage1b/snippets/src/main/java/org/wpilib/command3/button/CommandXboxController.java

@@ -18,6 +18,10 @@ public double getLeftY() {
     return 0;
   }
 
+  public double getRightX() {
+    return 0;
+  }
+
   public Trigger a() {
     return new Trigger(() -> false);
   }

examples/stage1/stage1b/snippets/src/main/java/org/wpilib/driverstation/XboxController.java

@@ -15,4 +15,8 @@ public XboxController(int id) {}
   public boolean getAButton() {
     return false;
   }
+
+  public boolean getLeftBumper() {
+    return false;
+  }
 }

examples/stage1/stage1b/snippets/src/main/java/sources/SpotTheError.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2026 FRCSoftware
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package sources;
+
+import org.wpilib.command3.Command;
+import org.wpilib.command3.Mechanism;
+import org.wpilib.command3.Trigger;
+import org.wpilib.driverstation.XboxController;
+
+class SpotTheError {
+  private final XboxController xbox = new XboxController(0);
+  private final Intake intake = new Intake();
+
+  void triggerBindings() {
+    // [triggerCapturedValueBug]
+    boolean aButton = xbox.getAButton();
+    Trigger aButtonTrigger = new Trigger(() -> aButton);
+    aButtonTrigger.whileTrue(intake.runAtThrottle(0.5));
+    // [/triggerCapturedValueBug]
+
+    // [intakeOnTrueBug]
+    new Trigger(() -> xbox.getLeftBumper()).onTrue(intake.runAtThrottle(0.5));
+    // [/intakeOnTrueBug]
+
+    // [intakeWhileTrue]
+    new Trigger(() -> xbox.getLeftBumper()).whileTrue(intake.runAtThrottle(0.5));
+    // [/intakeWhileTrue]
+  }
+
+  void triggerBindingsFixed() {
+    // [triggerCapturedValueFix]
+    Trigger aButtonTrigger = new Trigger(() -> xbox.getAButton());
+    aButtonTrigger.whileTrue(intake.runAtThrottle(0.5));
+    // [/triggerCapturedValueFix]
+  }
+
+  // [intakeClass]
+  class Intake implements Mechanism {
+    private final ExampleMotor motor = new ExampleMotor();
+
+    Command runAtThrottle(double throttle) {
+      return run(coroutine -> {
+            while (true) {
+              motor.setThrottle(throttle);
+              coroutine.yield();
+            }
+          })
+          .named("Intake");
+    }
+  }
+  // [/intakeClass]
+
+  void intakeClassWithDefaultCmd() {
+    // [intakeClassWithDefaultCmd]
+    class Intake implements Mechanism {
+      private final ExampleMotor motor = new ExampleMotor();
+
+      public Intake() {
+        setDefaultCommand(runAtThrottle(0));
+      }
+
+      Command runAtThrottle(double throttle) {
+        return run(coroutine -> {
+              while (true) {
+                motor.setThrottle(throttle);
+                coroutine.yield();
+              }
+            })
+            .named("Intake");
+      }
+    }
+    // [/intakeClassWithDefaultCmd]
+  }
+}

examples/stage1/stage1b/snippets/src/main/java/sources/SpotTheErrorPt2.java

@@ -0,0 +1,123 @@
+/*
+ * Copyright 2026 FRCSoftware
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package sources;
+
+import java.util.function.DoubleSupplier;
+import org.wpilib.command3.Command;
+import org.wpilib.command3.Mechanism;
+import org.wpilib.command3.Scheduler;
+import org.wpilib.command3.button.CommandXboxController;
+import org.wpilib.drive.DifferentialDrive;
+import org.wpilib.framework.OpModeRobot;
+import org.wpilib.hardware.imu.OnboardIMU;
+
+public class SpotTheErrorPt2 {
+  private final DifferentialDrive differentialDrive = null;
+  private final OnboardIMU imu = null;
+
+  class ArcadeDriveBug implements Mechanism {
+    // [arcadeDriveBug]
+    Command arcadeDrive(DoubleSupplier forwardThrottle, DoubleSupplier rotationThrottle) {
+      double forward = forwardThrottle.getAsDouble();
+      double rotation = rotationThrottle.getAsDouble();
+      return run(coroutine -> {
+            while (true) {
+              differentialDrive.arcadeDrive(forward, rotation);
+              coroutine.yield();
+            }
+          })
+          .named("Drive");
+    }
+    // [/arcadeDriveBug]
+  }
+
+  class ArcadeDriveFix implements Mechanism {
+    // [arcadeDriveFix]
+    Command arcadeDrive(DoubleSupplier forwardThrottle, DoubleSupplier rotationThrottle) {
+      return run(coroutine -> {
+            while (true) {
+              differentialDrive.arcadeDrive(
+                  forwardThrottle.getAsDouble(), rotationThrottle.getAsDouble());
+              coroutine.yield();
+            }
+          })
+          .named("Drive");
+    }
+    // [/arcadeDriveFix]
+  }
+
+  // [robotPeriodicBug]
+  public class Robot extends OpModeRobot {
+    private final CommandXboxController xbox = new CommandXboxController(0);
+
+    public Robot() {
+      xbox.a().whileTrue(printHelloWorld());
+    }
+
+    @Override
+    public void robotPeriodic() {}
+
+    private Command printHelloWorld() {
+      return Command.noRequirements(coroutine -> {
+            while (true) {
+              System.out.println("Hello World!");
+              coroutine.yield();
+            }
+          })
+          .named("Hello World!");
+    }
+  }
+  // [/robotPeriodicBug]
+
+  class RobotFixed extends OpModeRobot {
+    private final Drivetrain drivetrain = new Drivetrain();
+
+    // [robotPeriodicFix]
+    @Override
+    public void robotPeriodic() {
+      Scheduler.getDefault().run();
+    }
+    // [/robotPeriodicFix]
+  }
+
+  class RotateInPlaceBug implements Mechanism {
+    // [rotateInPlaceBug]
+    Command rotateInPlace(double angleDegrees) {
+      double targetAngle = imu.getRotation2d().getDegrees() + angleDegrees;
+      return run(coroutine -> {
+            while (imu.getRotation2d().getDegrees() < targetAngle) {
+              differentialDrive.arcadeDrive(0.0, 0.2);
+              coroutine.yield();
+            }
+          })
+          .named("RotateInPlace");
+    }
+    // [/rotateInPlaceBug]
+  }
+
+  class RotateInPlaceFix implements Mechanism {
+    // [rotateInPlaceFix]
+    Command rotateInPlace(double angleDegrees) {
+      return run(coroutine -> {
+            double targetAngle = imu.getRotation2d().getDegrees() + angleDegrees;
+            while (imu.getRotation2d().getDegrees() < targetAngle) {
+              differentialDrive.arcadeDrive(0.0, 0.2);
+              coroutine.yield();
+            }
+          })
+          .named("RotateInPlace");
+    }
+    // [/rotateInPlaceFix]
+  }
+
+  class Drivetrain implements Mechanism {
+    void periodic() {}
+
+    Command arcadeDrive(DoubleSupplier forwardThrottle, DoubleSupplier rotationThrottle) {
+      return null;
+    }
+  }
+}

src/config/sidebarConfig.ts

@@ -137,6 +137,10 @@ export const sidebarSections: Record<string, SidebarSection[]> = {
                                     label: 'Commands & Mechanisms, Pt. 2',
                                     slug: 'learning-course/stage1/stage1b/commands-and-mechanisms-pt2',
                                 },
+                                {
+                                    label: 'Bonus: Spot the Error',
+                                    slug: 'learning-course/stage1/stage1b/spot-the-error',
+                                },
                                 {
                                     label: 'Exercise - Kitbot Rewrite, Pt. 1',
                                     slug: 'learning-course/stage1/stage1b/command-based-kitbot',
@@ -149,6 +153,10 @@ export const sidebarSections: Record<string, SidebarSection[]> = {
                                     label: 'Exercise - Kitbot Rewrite, Pt. 2',
                                     slug: 'learning-course/stage1/stage1b/command-based-kitbot-pt2',
                                 },
+                                {
+                                    label: 'Bonus: Spot the Error, Pt 2',
+                                    slug: 'learning-course/stage1/stage1b/spot-the-error-pt2',
+                                },
                             ],
                         },
                     ],

src/content/docs/learning-course/stage1/stage1b/command-based-kitbot-pt2.mdx

@@ -2,7 +2,7 @@
 title: Exercise - Kitbot Rewrite, Pt. 2
 description: Rewriting the kitbot code in stage 1A to command-based
 prev: learning-course/stage1/stage1b/suppliers-in-command-based
-next: false
+next: learning-course/stage1/stage1b/spot-the-error-pt2
 codeRegionSources:
     default: stage1/stage1b/snippets/src/main/java/sources/CommandBasedKitbotPt2.java
 ---

src/content/docs/learning-course/stage1/stage1b/commands-and-mechanisms-pt2.mdx

@@ -2,7 +2,7 @@
 title: Commands and Mechanisms, Part 2
 description: Intermediate-level uses of commands and mechanisms
 prev: learning-course/stage1/stage1b/triggers
-next: learning-course/stage1/stage1b/command-based-kitbot
+next: learning-course/stage1/stage1b/spot-the-error
 codeRegionSources:
     default: stage1/stage1b/snippets/src/main/java/sources/CommandsAndMechsPt2.java
 ---

src/content/docs/learning-course/stage1/stage1b/spot-the-error-pt2.mdx

@@ -0,0 +1,72 @@
+---
+title: Spot the Error, Part 2
+description: A review exercise for stage 1B
+prev: learning-course/stage1/stage1b/command-based-kitbot-pt2
+next: false
+codeRegionSources:
+    default: stage1/stage1b/snippets/src/main/java/sources/SpotTheErrorPt2.java
+---
+
+### Exercise 1
+
+"Hello World!" doesn't show up in the console while the a button is pressed.
+Can you spot the error?
+
+```java #robotPeriodicBug
+
+```
+
+<details class="accordion">
+    <summary>Reveal</summary>
+    The scheduler never runs, so scheduled commands never execute.
+    Call `Scheduler.getDefault().run()` inside `robotPeriodic()`:
+
+    ```java #robotPeriodicFix
+
+    ```
+
+</details>
+
+### Exercise 2
+
+This command is supposed to drive the robot using the joystick's forward and rotation axes.
+
+```java #arcadeDriveBug
+
+```
+
+Can you spot the error?
+
+<details class="accordion">
+    <summary>Reveal</summary>
+    The supplier values are read once, before the command starts running.
+    The robot drives at the same speed and rotation forever, even as the joystick moves.
+    Read the values from the suppliers inside the loop instead:
+
+    ```java #arcadeDriveFix
+
+    ```
+
+</details>
+
+### Exercise 3
+
+This command is supposed to rotate the robot 90 degrees clockwise from its starting position.
+
+```java #rotateInPlaceBug
+
+```
+
+Can you spot the error?
+
+<details class="accordion">
+    <summary>Reveal</summary>
+    The target angle is read once, before the command starts running.
+    If the robot's heading changes before the command runs, it will rotate to the wrong angle.
+    Read the target angle from the IMU when the command starts running instead:
+
+    ```java #rotateInPlaceFix
+
+    ```
+
+</details>