Top runners:
Lost Ending
Dexter Ending
Artistic Integrity
Dragon Ball GT
Dragon Ball Evolution
Jurassic Park 3
Zilla '98
Star Wars Episode 1
Star Wars Episode 2
Jar Jar Binks
Much of the Star Wars Extended Universe
Naked Cartwheels of Liberty
Sutherland
And, most recently,
[spoiler]Makorra fanbase drowning themselves in their tears[/spoiler]
CAN'T FORGET THE BATFLECK
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The dexter ending wasn't bad, but it wasn't good either. I just sat there going 'meh' The whole season just lost my interest too :l However the Mass Effect 3 rage was the funniest thing I have ever seen on the internet. Months and Months of sustained outrage over the ending which I quite liked anyway.