It happens alot...as long as the admin keeps the releases updated (they have hacks ALOT against phpbb) everything will be cool
I run several phpbb and other sql run sites (on my personal webserver) and if its heavily loaded (by internet spiders and such) it can get boggy quick.
Definition of: 500 error
The HTTP error message returned to the browser by the Web server when it cannot continue to execute the code in a script. The error can occur for a variety of reasons, including a bug in the code, an undefined variable or lack of permission to a certain resource.