I have .. and I have ALWAYS stopped it! That's a good way to hurt yourself ... badly!
Here's how it looks (sorry, it's a staged presentation, It looks kind of crummy, but it DOES demonstrate the problem!)
Selfie Alert: It's difficult to get a good picture of your hands, when you've nestled your digital camera under your chin. I'm sure you're all familiar with that situation.
Anyway ... looking at the user-end of a 1911 (here, 2011 EDGE from STI), you can see that the knuckle of the off-hand thumb is sticking up into ... or at least near ... the 'glide path' of the slide.
I've seen a LOT of 'new shooters' use this grip, and it's perfectly acceptable if you're a revolver shooter.
But if you're shooting a semi-automatic pistol of ANY brand, make or model ... using this grip is terribly dangerous.
Because the recoil from any shot
Or (here) a 10mm.
Also, the same result (or similar) might be the result of putting a knuckle behind the recoiling slide of a .45acp, a .40S&W, or even a poodle-shooter (9x19). The power of the cartridge is immaterial; any gun can hurt you, if you don't respect its potential to hurt you. During 'normal' shooting exercises, shooters have time to ensure that their grip is consistently out of the 'flight path' of the slide.
But when shooters are presented with an entirely new shooting challenge ... when the time needed to draw, present the pistol and engage the first target may negatively affect the stage score, sometimes it's too easy for a shooter to get a little bit sloppy about his grip for the first shot. And that's all it takes to maim a competitor for life.
That's why I absolutely insist that shooters get into the habit of using a grip which protects their own physical safety, no matter how much they may hurry their draw and their shot.
Speed, Power, Accuracy. And above all .. SAFETY!
The thing is, people who have opted to take a class in Competitive Shooting (USPSA, etc) are supposedly experienced shooters who are looking for a more interesting competitive venue.
Usually, they are very skilled gun-handlers. But still, almost every month it's necessary to correct the grip of at least one student. (After all, it is essentially a class in Shooting Safety.) Which is why I start the "Live Fire Exercise" portion of each class by encouraging every student to observe and learn from the mistakes of others.
The Good News is, during the six years that I've been teaching this class, no student has ever experienced the pain and mutilation of having the slide remove Precious Body Parts.
The first offending student of each class is humiliated in the learning of the lesson, but the results of the difficult lesson is that they do, indeed, learn from each others' mistakes.
First Range Rule is: No blood on the range.
Second Range Rule is: We learn how to be safe.
Third range Rule is: We learn how to compete
Fourth Range Rule is: Shooting is fun!