It comes up all the time 100 line one-off scripts in python. You start off with 10 lines that you're iterating on. You move some snippet to a function, don't de-dupe your variable names from the global scope, and you get a bug where you thought "x" was scoped to the function, but wasn't actually defined in the function so you're using the global "x". Yes, this could be fixed by moving the global statements into a main() function, but this breaks your repl workflow and you have to resort to breakpoints in main().