I was doing some testing on singleplayer and I put 666 (don't ask why) TNT minecarts on a rail and held 1 totem of undying in each hand. Then I made them all blow up at once and after a huge BOOM I died.

I went back to the explosion site and retrieved all my items but the 2 totems were gone...

Could they have some sort of vanishing curse on them by default?

The sound effect was not played and there were no totem particles displayed, if they were used up then it was not apparent. I also should not have died since the combined explosions of the mine carts all went off at the exact same time.

if it was a big enough explosion, it could have lagged

I will do more testing...

Im doubtful that all the TNT exploded in the same tick.
I'd suspect its something like that caused you die, the subsequent explosions could easily lag a tick behind due to late activation.

interesting hmm,but yea i would think lag was involved with this.

@R4iscool1 Im doubtful that all the TNT exploded in the same tick.

This. It depends on how you activate them. If you have them stacked on a powered rail and and power it so they run into a block they don't all explode in the same tick. Do it with normal minecarts and you'll see they separate slightly
In this case the totem activates but you still die. You might not have seen the totem be used because you had so many minecarts. I used 5.

If you push the minecarts with a piston and drop them several blocks near you they all explode in the same tick and the totem will save you every time.

@NutjobBob Alright I will do that later and let you know how it went

My impression is that totems only practically protect you from certain kinds of damage. For instance they aren't going to be great against void damage, which is actually the number 1 thing I would like to have them for. God gear and pots will protect you against almost everything else.

I wish the totem was more of a keep inventory thing. Instead of dying, you wake up at your set home with all of your items intact. I think in the above example you could conceivably consume one totem per tick and die on the third tick, before the animation has time to play.

If a keep inventory item were put into vanilla minecraft, it would have to be a really hard thing to find or earn. Like if it was only available by crafting 8 nether stars around an eye of ender to make an "ender star" or something like that where you could use it up and would teleport home instead of dying.

Charged Ender Pearl? You put an ender pearl and a totem of undying inside a purple shulker box and put it outside and wait for it to be struck by lightning and then you have a charged pearl that teleports you home when you die

EDIT: And it has to be on a night of the new moon :P