The absolute worst for us was being stuck in the HM (and for the life of me I can't remember if it was at DL or WDW). All I remember is that we were stuck right next to the heads that pop up and scream. Well, everytime the heads screamed, the group of teenage girls in the doombuggy behind us screamed. And everytime they screamed, all three kids would cry even harder than they already were. For 10 minutes this torture went on until we finally moved, all of 5 feet, so we could no longer see the heads, but we could still hear them and the girls behind us. So after what seemed like an eternity, the ride was back to normal.
I don't know if my kids will ever want to ride on the HM again or if they are scarred for life. The oldest at the time came off a blubbering mess and the younger two closed their eyes and held their hands over their ears on every ride for the next two days we were there.