Tumblr’s editor turned this post into 

between the time I pressed “Create Post” (at which time there was no CSS gunk visible) and the time it was actually published (where some CSS crept into the post body itself).

(This happened both on Tumblr itself and in the wall post that the Tumblrbot made on Facebook — I immediately went and fixed the damage in the original post, but the Facebook publish feature is basically instant and doesn’t allow after the fact edits, so the CSS gunk is forever baked into that post and that’s why the screenshot shows Facebook and not Tumblr. Also, Tumblr apparently bakes the RSS feed with enough delay that my subsequent edit to fix the post made it into the RSS version, unlike the Facebook version.)

This problem isn’t incredibly common, but common enough to be annoying — maybe something like 5% of the time.