Randon pop-up warnings when I browse away from a Boards page

Tenshot

Randomly, when I'm logged in, Boards on IE11 will give me an "Are you sure you want to leave" pop-up, with the text 'false', when I try to navigate away from a page.

This could be clicking a post title in an index page, or clicking a navigation number (3 .. 4 .. 5 ..) at the bottom of the post. It doesn't happen that often, but just enough to be annoying.

I presume it's an artefact of some debug code that got left on by accident, and is triggering due to some obscure feature of my account.

I haven't noticed it on Chrome, but I don't use Chrome with boards much so I'm often not logged in there.

It's also possible that some weird glitch on my PC is causing this instead. On the other hand, I've seen it on both my home and work PCs, and I haven't seen anything similar on other websites, or when I'm on Boards but not logged in.

In the screenshot, I clicked on the 'Sticky' thread 2nd from the top to produce the requester, but it generally happens with any attempt to navigate away from the page.

Anyone else seeing this?