Topic: "Mark all topics as read" after timeout produce error
I've had this a few times, (very rarely), and I've tried to figure out the common thing I've done, and now I think I know what I have made 
Use the 'Show new posts since last visit' , keep the page (so you have the link for 'Mark all topics as read', I usually open up stuff in tabs), become inactive on the forum (so long that the last visit will change (I assume ), I usually do this by talking in the phone or something 
), click the 'Mark all topics as read' and voila, an error! 
Not really a biggie, just that I've got it a few times now and then and started to wonder why ... I'm not sure it happens on all hosts though, as this is the only place I've really noticed it on, perhaps I have a longer timeout/finish messages more quickly on the other forums I'm on ...
Happy hunting (if you feel it's a big enough thing to hunt down )