Well, I've just looked at the scene on the DVD, and this is what happens:
1) JLC fires, and is knocked back into a crate as the MAC-10 goes off on full auto. This actually takes just about the right amount of time to empty the clip - so that would be an accurate depiction of a slam-fire. Apart from the apparent recoil pushing her backwards.
2) After another couple of seconds, she drops it, and it starts firing again, despite having not been reloaded, upon impact with the stairs. This is not an accurate representation of a MAC-10 slam-firing.
3) on the first bounce it does actually keep firing back up the stairs as it tumbles.
4) it then fires off a short burst upon each impact with a step, mostly towards the approaching bad guys. But a few rounds still go vertically upwards. There are distinct pauses between each burst - not an accurate slam-fire representation.
5) It reaches the bottom, pauses for a second or two, *then* fires off a single shot back under the stairs, and then stops.
6) Amazingly, the approaching enemies have all been killed clean outright by this! And JLC isn't harmed.
So, I call myth busted... The clip would be emptied before the gun was dropped, there's no reason for it to go off on impact, and the amazing accuracy of the random bursts is so statistically improbable that they would pretty much have to be proof of divine intervention...
But then again, True Lies is definitely a fantasy flick, so why not? I guess Allah just wasn't on the villain's side., but secretly helping the heroes.