The worst is when a singer is singing a song he/she sang 20 years ago, and they can't hit the high notes like they used to, due to drinking, smoking, and aging.
I saw Poison with Warrant back in about 2001. Poison was great. Poison is one band that was never innovative, or super-talented, but they sure know how to put on a live show, and for straight up fun, they're good. Bret Michaels sounds as good now as he did back in the late 80s.
Warrant sucked. Jani Lane can not sing like he used to at all. It's nothing personal, actually I've met the guy (he's from my hometown) and he's a nice guy.
And he and Bret Michaels are about the same age, but apparently, Bret has taken much better care of himself and his voice.
Or they forget the words repeatedly..
*shuddering*