What happens is after you make a reply or a post, and then try to leave the page it gives you an error which just says null on it. But it's not really a problem because if you just close it, then you will still leave.
Which page were you visiting when you encountered this bug?
It can happen on all pages.
What can we do to reproduce this bug?
make a reply to a post, post it up, then press the back button to return to your previous page, or press a shortcut to go to a different page.
What was the expected outcome of the action that you took? That you would get an error saying null.
There have been numerous issues reported with the leaving page reminder from people visiting in Internet Explorer, I assume it's related to that. I'll make a note for the developers to also look into this when they investigate the other IE leaving page issue. Thank you for the report, in the mean time you may wish to use a browser like Chrome or Firefox when visiting this forum.