lexagraphical (spelling, or typos) can happen

trunLeft();
tuneLeft();

syntax errors can happen

turnleft();
turnLeft;
turnLeft()

Errors in logic can happen - The code compiles, but it doesn't do what you expect, or it will turn off if you run into a wall or pick up something that isn't there, or put down something you don't possess.

Consider the following:

It an be fixed by adding one method call.

Here is some code in need of debugging: