The biggest "oops" that I have seen was a purser that had some problems calculating the time difference between johannesburg and switzerland (in winter it is 1 hour, in summer there is none) and managed to wake up all the passengers 2 hours too early for breakfast!
I was a passenger on that flight, and I wondered about the early wake-up because i was quite sure you are not allowed to land in zurich before 6 o clock in the morning, and if you are woken up at 3 you will definitely land before 6. well, shortly after breakfast she had to make an announcement "i am very sorry and I understand that you are maybe a little bit upset, but i have to tell you I woke you up too early". OOOPS!