I'm a lefty and use the typical IPSC pistol two handed grip with the left thumb resting on top of the right thumb. If I accidentally press my right thumb up against the frame of the pistol it will hit that spot right on the slide stop and sometimes push it out enough to lock up the slide. I now consciously angle both thumbs out away from the pistol and have not had any problems since. I did flatten one of my slide stops once to prevent this from happening, but found I had the same problem as soon as I tried anyone elses pistol which wasn't also modified. I found it better to modify my hold as it eliminated my troubles in all cases then.