This is just a very general XenForo question, so it's not something we're having issues with ourselves nor is it something I'm trying to receive help for--they would have to ask, of course. I'm just using the forum below as an example.
Not as frequent anymore, but one of the XenForo forums I visit has been getting the blank pages with the "Unexpected error occurred" message for a while. When I attempted to visit yesterday around 2:00 PM, I received that error, and before that my last activity was Friday at 2:15 AM. However, before logging in 13 minutes ago, I checked my member card out of curiosity, and I noticed that my last activity was yesterday at 1:59 PM, which was when I last attempted to visit the site.
My question is why does activity or whatever still get logged sometimes when there's an error? Is the forum still running in the background and you just can't access it?