I noticed the same thing. Once or twice if I wasn't careful I could insert the Edge 500 into the Barfly a bit "cock-eyed" and it was not very secure - I could (and did) just pull it straight out of the mount. It never popped off during a ride, but I became very cautious about double-checking its anchoring after that.

Got a K-Edge for Christmas, and it clicks in just fine. I feel my Edge 500 is much more secure on it. The Barfly has now been relegated to the bike semi-permanently on the wind trainer.

Good luck,

KB