I edited a post made by another member and included a mention in the "@" form. After confirming, the topic refused to load. This was not just for me, but for all users. I disabled the extension and the topic loads, with all the edits made. There was no error, and it was just this one particular topic (not a necessarily large one either).
Just letting you know, I'm not sure what good my description of a problem can do without any sort of error code, but when the topic stopped trying to load I just got a blank screen.
I am using phpBB 3.1.2 - I am also using a theme called "Subway" if that gives you anything.