How A Convention Can Turn Bad...?

I decided this would be sorta fun. I've heard a lot about how one experience at a convention can ruin it, and I'm curious if any of our users out there have had things like that.

As an attendee, there's very few things that make it to where a convention isn't fun. I think the most annoying is when there's nothing interesting for me to do personally. I'm not a fan of AMV contests or anime screening rooms (yes, let me pay sixty dollars to watch anime all day) so when those are the only thing going on it is kinda disappointing.

Also, variety in the dealer's room is another thing I dislike or if each one is carbon copies of one another.

What about you guys? Specific experiences appreciated.