The Harlot has experienced this issue recently, too.
I have been working two socks at the same time - no, not that way - just two socks on two different sets of needles. One is on dpns, the other on magic loop. I often do this, to take as a prop to my sock knitting classes.
I've been working on the two, on and off, and as it often the way with projects I take to classes as demos, I'm not always paying full attention to them. Stuff goes wrong.
Take a look at the current pair - both almost finished.
See something odd?
The one sock (the upper one in the photo) has a nice organic stripey pattern all throughout the sock. The second (the lower one in the photo) has a rather nifty and much more regular stripe pattern on the foot. The red is pooling on one side of the foot.
All well and good, but they don't look the same anymore.
I did some thinking and some counting and came to a rather embarrassing realization: I'd missed a gusset decrease somewhere, and one sock has one stitch more than the other. One single lousy stitch can make this much of a difference. Wild, eh?
(Yes, that's Dexter helping me with the photoshoot. He is sad that the socks seem to be too big for him.)