Worst Crowd Reaction you've witnessed

Warbringer opening for Symphony X and Iced Earth in Charlotte, NC. Personally, I enjoyed them, but the crowd was giving them Hell the entire time. I actually felt sorry for those young guys.

~Brian~
 
I believe it was 1991 when I saw Believer at The Escape Club in Salisbury, MA. They were great but for some reason the audience wasn't into it. As would often happen at thrash shows the crowd opened a circle near the front of the stage for people to mosh, but no one was moshing. After a couple songs of us standing like idiots around this empty circle, I finally went in and sat cross-legged in the middle of it until the end of the show. I just hope Believer didn't interpret that as a criticism of them (they were great); it was the audience I was making fun of.
 
I'll go with the theme and mention a hostile crowd rather than a completely apathetic crowd (like the one I saw when Tommy Shaw opened up for Rush in 1988).

The most hostile crowd I ever saw was when The Cult opened up for Metallica during Metallica's 89 shed tour (I saw them at Popular Creek in the Chicago area). . While they weren't necessarily angry or particularly aggressive, they did start up loud "FUCK THE CULT" chants throughout their short set. The Cult played a decent enough set (I guess), but they were a bit too hard rock for the thrash fans Metallica was focusing upon at the time.

And it wasn't necessarily hostility, but when I saw Ozzfest 97 at Alpine Valley, Marilyn Manson wound up cutting their set short because the fans would not stop throwing stuff everywhere. I don't know if it was a hostile reaction to Marilyn Manson (who really weren't suitable for the late afternoon set they god--and they were sandwiched between Type O Negative and Pantera), or it was fans blowing off steam due to the long day.

One of the most interesting individual fan reactions I saw took place in 1991 when I saw the band Ratt. Midway through their set, Michael Schenker joined them on stage, and they closed out their regular set with a cover of Lights Out. A fan near me just went insane with joy--and yet he was also pissed that very few people around him knew the song. Needless to say, that fan was inebriated.
 
I saw the Melvins try to open for Gwar. Their first song, the guitar was set in front of the amps and left to feedback... really?

Eventually someone in the crowd started tossing firecrackers on stage. They played their set and then Gwar told them to play an encore. That's pretty metal... but they really sucked and the crowd wanted GWAH.
 
Godspeed You! Black Emperor opening for Nine Inch Nails last year. There was no crowd reaction whatsoever. I wouldn't be surprised if they were all asking themselves "what the hell did I just watch?"
 
I can't remember any specific instances of "bad" crowd reactions, but the closest I can think of is when Into Eternity opened for Redemption and Dream Theater years ago. I hadn't heard IE before but I dug the hell out of their set. Nobody stood at any point during their set (it was a large seat-only venue) and the band got polite applause at the end of each song. It pissed me off but I didn't want to be the only one standing, so I just sat as well. Almost everyone stood for Redemption though, and of course everyone got up for DT.
 
Godspeed You! Black Emperor opening for Nine Inch Nails last year. There was no crowd reaction whatsoever. I wouldn't be surprised if they were all asking themselves "what the hell did I just watch?"

I saw that show in Atlanta last year, and it was quite weird. I don't know the music, but it could have been one 30 minute song for all I knew. Definitely not the type of music that NIN fans would get into.