When I tried carrying my P-35 in my 1918 GI flap holster intended for the 1911 I found that the magazine catch button contacted the leather pad inside the holster. If I picked the holstered pistol up the wrong way the magazine would pop out, same if the holstered pistol was bumped against something while on the belt.
Carrying a P-35 in the wrong sort of holster may be where the concerns over the magazine disconnect came from.
I have read of a LEO, Canadian IIRC, losing his magazine while drawing his piece and being killed because of it. Thats got to be an extremely rare occurance. I've never had a magazine pop out of any pistol other than the instance I mentioned.
I suppose it could happen while stuggling over control of a pistol in a close encounter.
My FN 1922 has the heel type mag latch, so its unlikely the magazine could be disengaged accidentally.