The hardest thing I ever worked on was a calendaring system for scheduling ads on/off. The UI was great, but I just told them "oh, send me a cron format string for scheduling". It was easy for them to do, but very hard to handle on the back end. Cron format strings aren't fun to reverse engineer. Why did I do that?

File under "things programmers assume about time". It kept doing "weird" things from the user perspective and I ended up writing more tests for that subsystem than anything else I've ever done.