"The ones that are dangerous to carry are mechs with protruding fire buttons."
Yup, like this one. I had a battery explode in this mod back in 2012 when I put it in a pants pocket. Continuous pressure on the fire button caused the battery to quickly over-discharge and "kaboom!" I was fortunate that my pants were in a work locker and I was not wearing them at the time.
Nearly all mod explosions occur with mechanical mods when the battery shorts out. A torn insulator wrap on the battery or a coil short in the atomizer can cause the battery to hard short. A majority of battery explosions occur when a user transports loose batteries in a pocket or purse and they come into contact with keys or coins. "Hybrid" or direct battery mech mods have an increased risk to cause a hard short if the atomizer in use does not have a special extended 510 center pin.
Recessed fire buttons are a plus on mechanical mods, as well as those mods that have a locking feature for the fire button. If not, removing either the battery or the atomizer will break the electrical circuit; transport loose batteries in a plastic battery case. Regulated mods generally have multiple protection circuitry such as a timed auto-cutoff (10 - 15 seconds) of the fire button, the ability to turn the mod off electrically (usually 5 clicks), and hard short protection.
With all said, there is no battery which is totally "safe". Just know what you are doing at all times, don't be careless, and always practice safe battery practices.
Mooch's blog | E-Cigarette Forum