digtialWrite does a "non-atomic" read-modify-write operation on the IO port containing the pin that you're writing:
...............
It's certainly POSSIBLE that an interrupt could occur after the port has been read but before it has been written, causing pin changes done within the interrupt routine to be "lost", though I wouldn't expect it to be terribly LIKELY (depending on how often you call digitalWrite in loop().)
That's genuinely interesting.