Just could be my server being crap to be honest. I've been having issues for days with things running slow. But I just got shown my first error page, picture below. Most likely timing out or server host messing faffing about as usual.
Just to add more, my site is back up now and no error is being displayed (it's my stupid server). Some more information, not sure why this is happening? When I edit a node, be it a forum or category. Sometimes when saving the change it just takes forever and ends up showing me an error pop-pop box. But when I go on the forum and look the change was saved and made.
A few people have reported this issue, but it appears like it might be a PHP or MySQL bug. Whenever Kier has had it, a MySQL restart has fixed it. Unfortunately the reported reasons for it being triggered don't apply to us, so I don't know what we can do to prevent it.