Worst case of this I've personally had to fix was an airline where you couldn't purchase tickets if you lived anywhere with a negative time zone offset. They're a small European carrier so it was never a very big issue for them, and all the developers and management were in Europe so they never noticed it first hand.

Basically when you tried to purchase a ticket, their frontend would take the date-of-flight, and adjust it by the user's timezone. So a ticket for 2015-01-30 00:00 would become 2015-01-29 20:00, or whatever. The dates in the request did not match what was expected for your booking code, and 500 error.