That's what I get for ignoring that little voice in the back of my head...

Remember that post where I was almost done with the second sock?  (You know what's coming, right?  Cue Ominous Music.  Yep.)
I'd been knitting the foot portion of the sock, trying to ignore that little niggling voice in my head that kept trying to tell me that Something is Wrong.  "No, no."  I kept saying to myself.  "It's really fine.  The fact that this sock seems shorter than the first one is clearly an optical illusion, since I've knit the same number of rows of the pattern stitch from the point of the gusset to the start of the toe.  I know this because I've counted them 14 times.  Not that I'm worried that there is a Mistake somewhere or anything.  I just like compulsively counting rows of knitting.  Especially with my hands over my ears: one, la la la, two, la la la, three, la la la..."
Eventually, though, even I can't maintain that level of denial.  The second sock is clearly shorter than the first, despite my best efforts to stretch it.
Once I stopped looking just at the knitting after the gusset, and looked at the gusset itself, I found the problem.
Here are the socks, lined up sole to sole:

The one on the right is First Sock.  The one on the left is Second Sock.  See the problem?  Count the rows in the gusset - which is easy to do, since they're conveniently knit in stripes.  A fact I ruthlessly ignored in my denial.

Honestly, I could just cry.