there appears to be at least one way you can author CSS in posts such that it 500s any page your post appears on. we'll take a look at this ASAP, but in the meantime please make sure your post renders correctly in the preview tab before posting it. if it breaks there, it'll definitely break on the actual site.
if you broke it the first time (not naming names) that's ok, honest mistake. if you do it now that you know it's possible, we're gonna take away your posting privileges.