My 30 day karma just jumped over 40 points since I checked LW this morning. Either I've said something really popular (and none of my recent comments have karma that high), or there's a bug.
I've seen several unexpected increases on the order of 10 points over the last couple of weeks. (I don't remember the exact dates.) My guess was gradual undoing of prior mass-downvoting, but a Mystery Mass Upvoter is certainly another possibility.
[EDITED to add ...] A possible variant of the Mystery Mass Upvoter hypothesis: we have a Mystery Small-Mass Upvoter, who is upvoting old posts in Main (maybe because s/he is new here and reading through old material). But that only works if everyone affected has old posts in Main, which I don't think is the case.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.