If (either by sloppy programming [on the user's part] or some other mischance) an instruction requires more than sys.maxsize bytes to execute the REPL crashes. If I had to hazard a guess as to what is causing the problem I would suggest the trouble occurs when trying to compute the number of bytes to report in the MemoryLimit exception. (Note that I have no idea what is actually going on here...)

Here are the simplest steps to reproduce the problem I've managed to come up with in the short time I've messed with this:
>>> ''.join('a' for i in range(1024))
>>> _*2**29