Yes I think that was it. Couldn't remember the episode they did it on. We were expecting a special episode on Terrance and Phillip and it turned out to be Elian Gonzalez or vice versa. It's been a few years now and the memory starts to go ;)
The episode you're thinking of was when they were supposed to reveal who Cartman's father was after waiting the long break - and instead they played nothing but Terrance and Phillip doing fart jokes.
It was April Fools, and they thought everyone would be highly amused, but people got really mad! :~D
I think it was because the Elian thing went down right around then.
Well, there was the "Cartman's Father" cliffhanger episode. When they pulled the prank where they showed a Terrance and Phillip special rather than the second part of the cliffhanger... As I recall, they were getting death threats over it!
Mark