The problem was never fully identified, some thought it was juice getting into the back of the fire button, but either way it happened even when the mods were locked.
Im not sure if there were that many, the main issue was the fire button suddenly deciding not to work or miss firing during use which could lead to auto firing, either way, i never bothered to buy one and wont recommend them, the 20w had a bad 510 and the 40w has some possible 510 issues too it seams... the 30w has been ok but it too has some issues, it sometimes fails to fire when the fire button is pressed, the 20w had the same intermittent issue as well... the 30w seams to be the best of the bunch.
If you only just purchased it then you have a right to return it for something else at least...