I often get stuck using old versions of IE on scientific equipment. IE6 is okay, but IE5.5 and older are a nightmare for even basic things. Just checking my email can become a chore due to all of the bugs in it.
I've noticed the occasional PNG transparency too. They usually just look ugly, but if people start using them for situations in which you need to be able to see the content underneath the transparency it could become a serious accessibility problem for those still using antiquated web browsers.
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