If you've been using python for a long time, then you surely know that os.listdir() function returns an unsorted list of file names. I didn't care much until facing a situation, in which the sorted-sequential processing of files was crucial, and I could not remember whether the previous file processing were done in sorted order. Well, luckily they were. But hey, this is a little bit annoying, isn't it? Why os.listdir() returns an unsorted list of files? Would you like to find out?
Sometimes the bugs in programs do not depend on a programmer. For example, when a programmer expects a certain result from a built-in function, that would be the last place to check for bugs. But sometimes weird stuff happens, and it also happened to me.