Probably the intern who, while a decent and intelligent guy for the most part, thought it was increasingly hilarious to keep putting random cat GIFs somewhere in the product with each PR he made.

First time "ok, very funny, but you can't do that in production software, don't do it again."

The third time around the "joke" was wearing a little thin.

Eventually a script was written so that, every time he made a PR, he'd get "emailed" one of a few pre-defined messages from a random member of the team a few minutes later, telling him to remove it and stop pissing about...