I use the Glock 35 for USPSA matches and it is great!
The only time I've heard of a Glock firing while Holstering , was a few years ago and it had to do with Uncle Mikes Holsters. The Holsters were recalled and haven't heard of any since then.
Yes, I have heard of a Glock (G22 specifically) going off in a holster. It happened during a firearms qualification that i was at. One of the shooters had a windbreaker with a draw string on the jacket at the waist. Somehow the drawstring got caught within his holster and when he holstered his weapon, the drawstring got caught in the trigger guard and as he continued to place the firearm in the holster the draw string got taut and discharged his weapon.
Moral of the story, there are three safeties on every Glock. The trigger safety (the first safety), followed by the firing pin safety, and then the drop safety. No Glock can go off unless that trigger is pulled to the rear of the gun.
Please look on page 10 and 11 on the attached PDF that shows through rendered drawings how each of the safeties work on the Glock. Unless you depress the trigger, there is no way for this gun to go off.
Stupid me... here's the download http://www.glock.com/downloads/GLOCK_Gen4_en.pdf Sorry about that.