I didn't, but I will. I'm not trying to say they are perfect all the time. No one is. I'm talking more about the songwriting of their album (and I have no idea what Justin's role in that was)
Bad vocals happen though. There could be a million different reasons why they could've been thrown off (including sucking at live singing and being human and having a bad night!).
There were two videos of both singers being really awful. Also, being that I'm an asshole, I went to youtube and looked up more live videos o make fun of and 3 of the 4 that I watched were terrible as well. I think that they are good at putting on a show and getting kids so excited that the audience can't tell how bad the vocals are.