tl;dr: the "last activity" user on the main page/question pages is sometimes wrong. I haven't figured out exactly which conditions set this off, yet.
I just saw this on the front page:
Oh, great, another "there should only be one SE site" post. Except, wait... last activity 9m ago by Popular Demand? I'm Popular Demand. And I haven't touched that post recently. Have I been hacked? Let's have a look at this supposed activity:
There's nothing there. Looks like I have been to that question before, but it's been over two years. Is the system making another tired joke about how my username is a pun?
Oh, there was activity on that question: it was reopened. But you can't blame me for that... mmyers did it with his mod powers. (Well, actually... I guess you could argue that it's my fault... you'd be in good company, according to this out-of-context quotation.)
I actually rubber-ducked this problem out while I was writing this question, but the point is, under some conditions, the "last activity" user on the main page/question pages is wrong. I don't yet know if this is related to only reopens, or only mod reopens, or if it only affects previous editors, or what.
Expected behavior: if a mod reopens a post, the last activity is "reopen" (rather than "edit") and the attached user is the mod; if five regular users reopen a post, the last activity is "reopen" and the attached user is Community.