via @geoffreylitt on the other place:
@Replay has saved me hours of debugging pain this week, truly incredible technology...
tl;dr, record a trace of reproducing a bug, then step forwards/back at your leisure, retroactively inserting log statements. best of print debugging + interactive debuggers.
in this case, i did a thing twice and it broke one of those times.
i finally tracked it down by noticing a line that had only run in one of the two executions
i've always been a big fan of time travel debuggers as an abstract idea, but it makes me so happy to see that they are *actually useful* when you're trying to understand what a program does!
Dan Miller