How to troubleshoot an assertion failure

I added a stack backtrace. It's not a real interpretation of the stack, it really just looks for addresses in the stack that are within code and then looks up the symbols, so there are spurious entries. The result is:

0x40209361: jsAssertFail at ??:?
0x4020d21f: jspeFactor at ??:?
0x40202e2c: jsvLockAgain at ??:?
0x40202e56: jsvLockAgainSafe at ??:?
0x4020c598: jspeFunctionCall at ??:?
0x40201190: jslSingleChar at jslex.c:?
0x40201b44: jslGetNextToken at ??:?
0x4020b6c1: jspeBinaryExpression at ??:?
0x4020d427: jspePostfixExpression at ??:?
0x40202024: jslMatch at ??:?
0x4020b6ec: jspeAssignmentExpression at ??:?
0x4020bcd4: jspeExpression at ??:?
0x4020d389: jspeFactorFunctionCall at ??:?
0x4020e355: jspExecuteFunction at ??:?
0x402055ea: jsvIsStringEqualOrStartsWith at ??:?
0x402044f9: jsvLock at ??:?
0x4021045c: jsiExecuteEventCallback at ??:?
...

The fact that this happens in an event callback makes sense to me. Dunno where to go from here though.