A problem in teaching with interactive notebooks (e.g., #Jupyter): students tell me, "I typed exactly what you did, and mine doesn't work." Now, sometimes it's not *exactly* the same (e.g., = vs == or the wrong kind of brackets).
Other times it *is* the same, but they had previously run some erroneous code that redefined some function or variable. Perhaps they redefined max to be a specific list rather than the built-in functions that returns the largest element in a list.
That code is no longer in the notebook since they edited it trying to fix their mistake. Restarting the runtime environment usually fixes this, but it's a nasty issue to track down.
@trinsec It does seem like almost every computer problem comes down to some variation on that idea. It works because maintaining state is tricky -- perhaps an argument for functional programming.
@peterdrake So basically you'd have to ask them every time at the start of troubleshooting: 'have you tried turning it off and on?'