I can remember a tank in Foundry HM in the fight against Revan once (pre 1.2) when he said "why aren't you interrupting this ability?" as the healer there I answered "I'm healing you, I don't have the time to watch his castbar as well" back came "no, not you, the dps", I said on that "they're both mercs they don't have an interrupt"
good old times
Use to run PUGs after work every day with a friend and guild mate, one week we got on a roll The False Emperor like 10 times on running 2 PUGs a day on two different toons. (me dps/heals, friend Tank/heals/dps). We were all fresh 50s at the time and I will still learning how to play a MMO. However, being someone really worried about causing a wipe I studied the instances and the fights. Everytime, my friend would ask does everyone know the fight and always everyone would say yes. When he was on his shadow tank, we never had a problem, but when he was on his commando either as heals or dps, we noticed no one, but one of us, would ever interrupt Unlimited Power, so we got to playing a game with each other waiting until the last possible moment to use our interrupt and scaring each other to death. 10 times in a row and people that said they knew the fight never interrupted what was back then a sure wipe. Really good learning experience for me as a healer as it taught me to use focus target and to watch what others were doing in a instance and what was going on in a fight. Before I only focused on myself getting out of AoEs and others peoples health bars. Still laugh when I get talked into healing HMFE for someone and I still wait until the last second to interrupt Unlimited Power though now it just isn't the same since it will not wipe the group.