For me, the worst reaction I've seen to a support act is easily Chicks on Speed opening for the Red Hot Chili Peppers at their Hyde Park gigs many years ago now.
To be honest, the early signs weren't good. Not only were they introduced by one of the members of the headlining band (never a good sign: I later found out that it was because they'd been booed off the stage the night before), but his main compliment was that they made all of their own costumes and they had some fantastic stage props.
Still, the crowd were polite enough to give them a couple of songs to convince them, but their odd brand of "quirky", naive anti-guitar electro-pop (one of their songs was even called "we don't play guitars", which they sang quite defiantly) and their novelty costumes (that looked like home-made primary-coloured superhero outfits) didn't go down well with a crowd who had turned up to see RHCP. Neither did the intermittent tuneless electronic screeching which seemed to emanate from a small wooden box.
After the first couple of songs, the crowd started booing at the end of all their numbers. Only when the band said "This is our last song..." did they receive a huge cheer and applause from the audience. At this point they launched into a huge tirade against the crowd, saying that we were being closed-minded and mean, and basically telling them off for being so rude.
(I did enjoy their closing broadside though: "This is the sound of the future, and if you don't like it you can fuck off back to Croydon!". Charming.) It was also pissing it down, which probably didn't help.
I don't think I've ever seen such an unpleasant, antagonistic relationship between a band and a crowd, and to see it at a massive venue like Hyde Park was just bizarre.
Luckily, James Brown came on soon afterwards, the weather brightened up, and the shittiness was forgotten.
I really think these things are often more a case of mismatched bands rather than the quality of the support act though. I remember seeing Aerosmith years ago and their support act being Shed Seven. I think I was one of the few people who enjoyed both, but the crowd on that occasion didn't dislike the support band, they were just a bit baffled by them, as they weren't really to their taste.