In the worst-case scenario, the simulator may enter an invalid state where some invariant (assertion) no longer holds. (This can indicate some corruption of data structures, for instance.) If you see such an internal simulation error (ISE), calmly back away from the computer and file a bug-report.
Internal simulator error: in some_function at some_file:line:
*** Please submit a bug report including version
*** "HACKT-...", VERSION,
*** (preferably reduced) test case, steps to reproduce, and configuration,
*** if appropriate, to <email@address>.
*** This program will now self-destruct. Thank you, and have a nice day.