I've seen enough spoof horror or science fiction films to know to stay away from them (Killer Klowns, Mars Attacks! etc)
But probably the worst movie I've seen was Battlefield Earth. I'm a sucker for SF (even, in the past, spoof SF) and I thought it couldn't possibly be as bad as reviewers said. It was worse.
Even worse than In the Realm of the Senses, which is saying something. Incidentally, In the Realm of the Senses was shown in a local art house movie theater at the same time as In Bed with Madonna. I wished I'd chosen to watch the Madonna movie instead, which shows how bad it was.
I also refuse to watch anything with Adam Sandler - he can act but it's a crap shoot whether any particular movie is watchable. Also, any movie with Sandler and Rob Schneider together is unwatchable.