A patron emailed me a message containing the string “(<-”.
The system apparently treats this as HTML or some other kind of special code and it actually breaks the Patreon messages page and cuts off the rest of the message completely. (I only saw it in the email notification).
Also, I tried to follow the instruction on the following page to report this bug:
But the “Email Patreon Support” button leads to a form that allows only very specific issues and “reporting a problem with the site” was nowhere to be found.