The Keristrasza one I think I completed on all chars at once because I left them sitting at the "end" point.

My worst one I can remember off the top of my head is in feralas - turning in the muisiks or whatever and he takes them, goes and does a dance by the water trough, waits a minute or so, then comes back, waits another 15 seconds, then lets another person turn the quest in.