Jumps that shouldn't reach target platform, but they do.
Surviving devastating blow from boss with 1/2 of normal minumum hitpoints.
Actually, the first is almost kinda simulated, with edge-grabbing and
toe-grab jumps. Whether you count them as legit (in-game supported) miracles or not is debatable, but from the player's perspective pulling one off is definitely a miracle. For an example of the second, check out EarthBound, which occasionally causes mortal attacks to leave your character standing with 1 HP.
Otherwise, I'm gonna have to go with the esteemed Mr. Sinclair here. Honestly, during normal conditions (and in most genres), I would prefer the parameters to be set in stone, and never vary unless the alternative is death. Otherwise you have players taking those advantages, and then feeling let down once the game decides they've ran out of luck and doesn't give it to them. You give players a 1/4 chance of an inch, and you better believe they'll take the probable mile.
Not that the idea is bad, if anything, it's a nice way to add a bit of balance by skewing the advantage to the player when it
really matters. I guess that the problem lies in making it so infrequent that it can't be exploited. And at that point, why bother? As stated above, the RNG or subpixels could be particularly generous and make something that, to the player, seems like a miracle. But I don't know.