What pissed me off more than anything as a kid was when they canceled the saturday morning Sonic cartoon, the one where Robotnik was an evil, badass dictator instead of a functionally retarded person. Unlike the weekly show most people remember (the one with Scratch and Grounder and the Sonic Sez segments that will forever be immortalized in youtube poop) this one actully had a storyline, and a pretty damn good one for a kids show. Also it had the coolest intro of any kids show ever.
The last episode ended on a huge cliffhanger for the third season, which never came and pissed me off to no end.
I think it was canceled because it couldn't compete with Power Rangers, so naturally ABC replaced it with something that could... a f*cking Free Willy cartoon that nobody liked. F*cking idiots.