I think the worst solo I've seen live wasn't a solo as such, but you'll see what I mean...
I saw a jazz combo down at the Old Duck** in Bristol one night, the bassist had a nice old Jazz and my first impression was that he could really play. He took a solo, which was very impressive, but it became apparent that he just didn't know when to stop. He just kept doing it... he widdled away all over the sax solo, he even kept on in the drum solo, seemingly oblivious to the fact that everyone else except him had stopped. After a couple of songs of this inability to STFU, my opinion of him had totally changed, and I just wanted him to shove his nice old Jazz right up there and sod off.
** as all you Brizzlers will know, its actual name is the Old Duke... but I had a Brazilian colleague some years ago who called it 'Old Duck' and it's kind of stuck.