Clients installing software from a test system to a clustered live production system and finding out it doesn't work the exact day they want to put it into production.
Yes, you do have to plan in testing, and yes, you do need to ask us to do that. Yes, in advance, not at the very last moment when we don't have time to make fixes and test them. No I am not Jesus, although I'm pretty much the next best thing. No it probably won't be working tomorrow. Yes I know the world cup finals just passed so it's really really really important last week. No I cannot reverse time even if that would've made a difference. Yes, you do need to indicate well in advance when you want to have something live and what you expect from us. No I don't care you're busy when all it takes is writing a short email. Yes this could've been avoided if you simply wrote that mail a couple of weeks ago.