I just logged back on after a brief absence from the site (a few days) to find I seem to have been genuinely karmassassinated. As far as I can tell, every comment I ever made has been downvoted, which was apparently enough to put me from 1200+ karma to -80

Wow! That's the first serious karmassassination I have become aware of. People do pissweak karma assassinations every now and again but I haven't seen a serious 1,300+ kill. I'm frankly surprised anyone bothered.

(although oddly, the "last 30 days" thingy claims I only got -148 karma; maybe it's maxed out?)

The last 30 days measure is only for votes on comments that were written in the last 30 days, not votes received in the last 30 days. The initial implementation included all votes received but had the effect that certain users could sometimes stay on the Top Contributers, 30 days list even if we didn't log in.

I understand it's possible to check who downvoted comments? It was probably a generically-named sockpupuppet, I guess, but still.

Possible, sure. I'm not sure how convenient it is. It may require direct database access.

EDIT: great, I apparently need to wait 8 minutes because I already commented. Did not know low karma did that.

Now usually when I see people complain about this I think "Working as intended! Downvote!". But this is a clear exception. Assassination is certainly not intended. I hope someone is able to sort this out for you.