There's an awful lot of less-critical stuff that users have tracked down themselves. A few random highlights:
- The mobile and desktop sites are completely separate products with vastly different behavior. Some privacy features (relevant to both) can only be accessed on one, some on the other. Tags are rendered in all-lowercase on mobile, but as written on desktop. Block quotes on desktop render as enlarged-font cursive on mobile, for some awful reason.
- Tumblr support(s/ed) font coloring, with no documentation of that fact. You enable it by using the HTML editor and picking among color tags with Friends-themed names like "Monica Pizazz Orange". Oh, and the preview feature won't honor the tags, but actually posting will.
- NSFW content is flagged even in drafts, but if that content is reviewed and approved, it's automatically posted publicly, not returned to drafts where it started.
- Tumblr's desktop sign up page use(s/d) semi-random images from the site as backgrounds. Yes, they did serve cartoon porn to people trying to make accounts.
- Certain posts were impossible to view. Tumblr accounts can have their own themed pages, or simply be popup sidebars over the main news feed. Tumblr "read more" content hiders took users from the news feed to the poster's account - if that account was in popup format, a readmore opened from the wrong location would simply force a circular redirect.
- All Tumblr links are actually pushed through a site-specific forwarding system to track users. As a result, Twitter and many other sites are inaccessible because they view all link clicks as bot traffic from a "single source".