The easiest way I've been able to reproduce is the following

1. Have a uniform or vest in a vehicle or on the ground, and any gun.
2. Have at least 2 mags in your backpack for the gun you have equipped, with none in your gun.
3. Drag the mag to the weapon to reload.
4. Spam right click the uniform or vest that you originally had on the ground.
5. If done right, there is a chance (seemingly relative to server load) that it deleted one of the mags inside of the backpack

Video: (Done on KoTH US#4, reproduced on US#2, Olympus, and GrandTheftArma)

https://youtu.be/WJ9zo2Y_qq8

This happens regardless of what types of event handlers you have on your client, or player. I removed them all and stopped all active scripts to make sure it was an issue with Arma and not something that I was able to fix locally.

Arma version
2.12.150779

Still happens on Profiling
2.12.150854