My worst experience has good to be Chessington in 2012. Although I was with someone with an exit que pass, we got an awful ride count due to the ques for the exit pass(As it was packed), and basically everything breaking down throughout the day. On top of the breakdown, there was thunder and lightening a lot of the after noon closing all the rides, apart from Tomb Blaster and Bubble Works.
After the day, it was on there website saying that you could use the tickets again to go back, but that was no use to me as I'm a merlin annual passholder.