While it is "possible" to hit the mag release, I've never had it happen inadvertently.
I have.
A minor fix to the holster prevented it happening again, but it is possible. Remember that carry holsters are often designed to minimize bulk, which means the button isn't protected by extra layers. In my case the holster was fine with stock grips, but would allow the botton to be depressed when I swapped to slim grips.
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Another option for dropping the magazine is to use the off-hand, I experimented for a while with my PPS that way and in the end decided that strong hand middle finger was the best way for me.
For off-hand release, you'd pinch and pull the paddle when you loacked open, sweeping the empty out as your hand went down to the belt for a reload. The downside is that is isn't the same if you're reloading before going empty.
What works
for me is to use the middle finger of the strong hand, with my thumb pushing on the other side of the gun. When I have a good grip on the reload mag in the left hand, a very slight shift in right hand position dumps the mag as I bring the gun in a bit for the fresh magazine. The advantage is twofold:
1- reloads are always the same, regardless of whether I'm dumping an empty magazine or a partially full magazine
2- I can (and do) use the exact same motion to reload button-release guns, because my thumb is mashed right where a button would be. (really high button releases like the CZ70 and Walther PP-series are the exception here, and obviously a heel release isn't conpatible with this method ... so I don't carry high-button or heel-release guns)