Well I didn't see how it landed but it fell out of my pocket and landed on a metal surface. I would guess the Protank hit first and it broke the thread going into the 510 connection. It still vaped but when I removed the tank the threads broke off. It didn't stop me from buying another one (actually 2) so I still use a Protank but just need a bottom to have 3 up and running. I really like the fact that it broke and didn't damage the 510 connection.