Internal Error: Sub-system: MEM, mem_segment.cpp

I only have a modest 3GB machine ...

So you must be doing a big design?

Looks like the synthesis gets caught in a loop, and that is a result of your source code. Were you able to completely compile your project at an earlier stage before the error popped up?

Until now I have always been able to either just restart Quartus, or to backtrack on my code and then spot the error.