I had one mag for MP5 that had a serious malfunction. I sent it back to Beta, and replaced it. No problems since.
The issue I had was the bolt carrier bit into the top "ramped" dummy round and pushed it out of the feed tower. The gun jammed, with a couple dummy rounds up in the action of the weapon. I reassembled the mag, and test functioned it by hand, and the thing kept falling apart due to the same "biting."
When I sent it back in, it got lost between the USPS and Beta... Neither USPS nor Beta would take resaponsibility, so I had to write it off and buy a new one. Lesson learned: don't rely on delivery confirmation... send things of value insured. That requires a signature upon delivery, and is relatively inexpensive.
I'm looking forward to .308 mags becomming available.