Have you seen negative voltage on cells?

I always thought it would be some kind of urban legend.
The 10s battery of my bottle pack didn't turn on after the winter even though it was connected to the charger (which was not plugged into the wall outlet *doooh*).
So i opened it. It showed some 20V.
First, second cells 2.8V. Then third and fourth -0.4V(!!) then 3.3V and the rest 2.7-2.8V.
How can this happen? I don't plan to do some kind of stupid reviving, and will replace the dead cells. But still i'd like to know how this could happen?!

It's not an urban legend. I've seen it multiple times on old, dead packs.

I've see -2.69V on one cell group.

https://syonyk.blogspot.com/2016/11/trek-transport-battery-teardown-with.html

If one cell group is weak compared to the rest, it'll have lower capacity. Say, 900mAh vs 1500mAh or something. As the pack drains (usually a BMS with a high idle draw is at fault), current flows, and that group hits 0V - but the rest of the pack still has more than enough voltage to drive current, so it keeps getting current forced through, and it goes negative.

You're correct in that it's quite scrap at that point.

Couple of years ago or more, it happened to one of the EIG NMC cells I use, in my old lighting pack, which had been damaged by overdischarge (to zero) years before, then accidentally left running the lights again. I forget how far below zero it went, but it was puffed up, which has never happened to any of these cells so far, to me. The other three cells were not too far discharged to save (the headlight that draws a few amps was still lit, if dimly), but they'll never be the same. :/