Be on the lookout
Working with a touch sensor, students sometime continue to use timed blocks (e.g., On AC For 3) in conjunction with a sensor watcher, rather than giving general instructions and have the time be determined by sensor input. It is important to help students understand that without sensor input, the behavior is fixed and, therefore, predictable. With sensor input, behavior is determined by input and therefore is unpredictable.
What you can do: Ask students whether they must predetermine the length of time the robot should move, or whether that amount can be left open and vary according to sensor input (e.g., Do you want it to move for 3 seconds every time it runs, or do you want that to change depending on how far it is from the wall?) The key here is the word “depending”.