Not sure if this link will work and the vid I found is of a RC lipo pack exploding but I imagine that the batteries we stack would go up in the same manner(except contained in a tube, like a pipe bomb) and even an event reduced in scale going off inches from someones face... I'm thinking that it would be like a little grenade, forcing pieces off of a mod to go flying like shrapnel, can this be considered proof of the danger's of stacking batteries?
Lipo explode - YouTube
ehhh.....Yes and no. I've seen vids of cylindrical lithiums "exploding". I've had a lipo pack "explode" on the charger. LiPos go up more violently. The OP's question wasn't so much about the effects I think, but of the risk of it happening in the first place. I'm not sure "explosion" is the right word anyway. I's more of a vent, ignite and burn. It's a process that takes some seconds and with cylindrical batteries in a mod, you get additional warning time. If it's in your hand, it will heat up from the venting and allow you time to at least drop it.
It's a bit hyperbolic to compare it to a hand grenade or a pipe bomb though. Those are instantaneous explosions with far more force and energy than any battery you'd likely find stacked in a mod. Unless a mod's casing is exceedingly flimsy, the weak spots will be the connector and switch. Those will fly off and release pressure before the casing splits, let alone fragment or do anything resembling an "explosion", a word that's used too loosely around here, IMO. If it's got decent venting, nothing will fly off but you could still get burned pretty bad if you weren't sentient enough to drop it. Flame can also be released, burning nearby objects.
There are videos of stacked batteries venting in a Silver Bullet. Nothing happened. Nothing flew off. The switch was displaced a few millimeters and it emitted a lot of hot gas. Another member here reports of having stacked batteries "explode" in his Silver Bullet without it being destroyed, albeit it was too hot to touch for an hour.
In a lesser and/or unvented mod, certain pieces can go flying off, but words like "explosion" and "shrapnel" are a bit strong for what is actually likely to happen in the real world, IMO.
None of that argues for stacking batteries. It's almost totally unnecessary nowadays to stack batteries in a mod not designed for it. Even without any spectacular explosion and flying shrapnel, hot gas burns are serious enough not to risk it.