When you did `tu <search term> it would search Tumblr for posts with that term or tag, pick a random one, and link it in chat. It'd also reblog the post to its own account and add one of a dozen or so random silly generic comments. All this was back before the Great Tumblr Porn Ban, so much of the time it was used to find good porn.
Someone eventually realized that lots of Tumblrites would write posts with the tag "Don't Reblog" and get very triggered when the bot reblogged their post. Things along the lines of "`tu suicide tw don't reblog" were a source of great lulz for the Wetfish community of yore. People started tagging their posts as "dont rebl0g" but we just changed our search strings.
The ask feature was also hooked to IRC, so that all the messages these bemused-to-seriously-triggered Tumblrites would send to the account could be laughed at in real time.
Eventually someone noticed that Tumblrina's account looked really fishy, being about 50% porn and 50% horrible shit tagged with "don't reblog." They posted about it and dbladez said he was shutting down the bot until we figured out who made the post. This triggered a massive witch hunt trying to figure out who THE MOLE was. Some people were quick to blame specific people in the chat, but it is also possible that there was no mole and somebody simply found the source code on github.
We may never know.
<tumblrina> [New Tumblr Message] Anonymous writes: Fuck off!
Sources