I don't understand this, but it happened.

I'm doing an animation, 300+ frames, (not that it matters how many).

I had only a base layer, which I was happy with, stuffed as it was by toil with sweet keyframes.

All that was left was to do some arm movements, which were totally absent, (the arms were static so far).  So, I create a new layer and set it to add (instead of replace).  Well, I dabbled a few minutes then reached a point where I decided to delete the layer and start over. Well, after I deleted my new layer, my figure's arms were in zero'd position, not where they were before, in a neutral standing position (hanging at the sides). 

That means, deleting an animation layer affected the keyframes of the layer below!
WTH!
huh? Am I insane, or is THAT insane? Just exactly how does this cobbled-together legacy village do it's adding and replacing? What should I do next time?

clarification: The static position of the base layer was as if hanging at the sides.

"Cobbled-together legacy village" is a beautifully apt description of Poser,
and the Animation Layers function is cobbled with chewing gum and bailing wire.

In other words, it doesn't work right.

heh heh, yeah.
Sometimes it feels like barbed wire, and I just want bailed out :(

Ockham, reading your comment that Animation Layers do not work right was very disheartening, knowing how much of an expert you are. I'm just getting in to them now.

Is it a case where the problems are consistent? In other words, if you make sure you don't do thus and such, thus and such, and thus and such then you should be ok. Or is it the case that there is no rhyme or reason to the problems, sometimes it works and sometimes it doesn't?

Its a bug in Poser, they know about it, but they don't have it sorted out yet...
Instead of deleting the layer, just switch it to add, then turn it off....