Software Gotchas: Inverted Drives, Scheduler Stalls, and Reading the RioLog

Most rookie software bugs are not subtle algorithm errors — they are a short list of the same gotchas, repeated every year. Recognize them on sight.

1. The robot spins instead of driving straight. One side is not inverted. Your two gearboxes face opposite directions, so a positive command drives them apart. Call setInverted(true) on one side (usually the right). If the robot drives backward when you push forward, you negated the wrong stick — remember Xbox stick-forward is negative Y, so arcadeDrive wants -getLeftY().

2. The robot looks dead but Robot Code is green. In command-based, you forgot CommandScheduler.getInstance().run() in robotPeriodic(). Without it, the scheduler never ticks — no default commands, no button bindings, nothing. This is the single most common command-based bug.

3. A mechanism fires once and never again, or a command never ends. Your command has no isFinished() end condition, or an autonomous step lacks withTimeout. A command that never finishes holds its subsystem forever and blocks every later command that requires it. Either return true from isFinished() when done, or wrap it in .withTimeout(seconds).

4. 'Loop time overrun' warnings. Your periodic code is taking longer than the 20ms loop. The usual culprits: System.out.println spam every loop, or blocking calls. Print sparingly and move heavy work out of the periodic loop.

Read the RioLog like a pro. The RioLog (or Driver Station console) is your primary debugger. After every deploy, watch for 'Robot program starting'. A Java stack trace there means an uncaught exception — read the top line of the trace for the exception type (e.g. NullPointerException) and the line number in your file, not WPILib's. A NullPointerException in your constructor often means a device failed to construct (sometimes a CAN ID typo) and your code crashed before teleop.

Instrument with telemetry. Push values to NetworkTables and watch them live in Glass or AdvantageScope — both are programmer debugging tools (not driver dashboards). Plot a stick value, a motor output, and a sensor reading on a graph; if the stick moves but the output is flat, you have isolated the bug to your command logic rather than wiring. AdvantageScope can also replay a saved WPILib data log after a match so you can debug a problem you couldn't reproduce in the pit. Logging beats guessing every time.