Forum OpenACS Q&A: Re: Fatal: received fatal signal 11 - new error after years!
One occurs when threads exit nearly as fast as they are started, which generally requires maxconnsperthread to be set very low - around 20 or less, which you wouldn't have on a production server - and high request rate, like benchmarking. This problem is reduced but I'm pretty sure not completely eliminated in aolserver HEAD.
Another comes up if you set an redirect for 500 error pages that doesn't exist. It looks for the error page, tries to redirect to not found, gets an error, looks for the error page, tries to redirect, etc, until it runs out of stack space and crashes. This problem is fixed in aolserver HEAD.
There's another one related to mmap failures (NULL is not MAP_FAILED) but I don't recall the specifics of that one.
A stack dump from a core file might help give a better idea.