I loathe it when I look for obscure TV shows online -- ones that were filmed in 4:3, naturally -- and end up only finding episodes which have either been stretched or cropped into widescreen.
What the fuck is wrong with the people who do this? Do they really think that seeing characters turned into squat trolls or having the footage blown up until everything looks fuzzy and half the picture is missing is preferable to having two black bars on either side of a screen?
[JEDIT]
Also, fuck YouTube (and other online video players) for not having an option to change aspect ratios.