For those of you who've never experienced the joy of PowerBuilder, this could often happen in their IDE due to debug mode actually altering the state of some variables.

More specifically, if you watched a variable or property then it would be initialised to a default value by the debugger if it didn't already exist, so any errors that were happening due to null values/references would just magically stop.

Another fun one that made debugging difficult, "local" scoping is shared between multiple instances of the same event. So if you had, say, a mouse move event that fired ten times as the cursor transited a row and in that event you set something like integer li_current_x = xpos the most recent assignment would quash the value of li_current_x in every instance of that event that was currently executing.