I had this happen on a 510 L2. The battery stuck. I figured it was defective and sent it back to the supplier. I heard others had the same problem with the 510 L2.
I don't need an atty sticking and staying on any time I use it, especially when I could have walked off after taking that puff. Luckily, I had it standing up in a glass at the time, so no damage was done.
I had just taken a puff and put it down. A few seconds later it suddenly started smoking and I heard the sizzling and realized it hadn't shut off after I had taken a puff. The LED was NOT lit to show it had stayed on.
So I figured the button must have stuck. I have never had it happen with an original 510. I don't want an atty that has a button that sticks. It seems dangerous to me. What if I had left the room? What if I had laid it down next to a papertowel or magazine?
You would have to be absolutely SURE it shuts down after every puff you take. I like my home. I don't really feel like burning it down.
Debbie
Added: I wasn't able to discern 2 LED's on my 510 L2 either. Other than the larger bridge inside the atty, the whole ecig appeared to be the same as an original 510 to me. I knew, however, that it was a 510 L2 because that was what I had purchased (thinking at the time it was just a newer model of the original 510).