Just seeing this thread now.

I actually had this issue with a complete lower from a very well known manufacturer on this forum.

It surprised me because it didn't do it every time, and the first time it happened I thought "I didn't just pull the trigger twice??" This was very difficult to diagnose because it wouldn't happen consistently and would often happen during rapid strings of fire. One day I was doing slow fire and noticed it fired as I released the trigger and knew I had a problem.

Luckily this was taken care of by the manufacturer but I can certainly attest that it does happen even to the very best manufacturers.

Great thread.