You know how it is - you're chugging along on a second sleeve, making decent headway, and about a third of the way through it or thereabouts, you hold it up in front of you, and smile a satisfied smile because you can see the end of this sleeve in the not-terribly-distant future, and then you hold it up against the completed first sleeve, and you keep smiling at your progress.
And then you count how many decreases you've already done, but the number doesn't seem to jive properly. So you count the decreases on the completed sleeve, with the smile sort of frozen on your face, and that's not working out right either. You look more carefully at those two sleeves - or, well, sleeve and partial sleeve - and notice that your decreases don't seem to line up.
And then the smile is gone, and you're counting the decreases, and yes, the numbers are absolutely wrong, so you start really closely examining how many rounds are between those decreases, and realize that you miscounted or something when you picked up the first sleeve to finish off the last two decreases, so the last two decreases are placed every 9 rounds, but the first ten are placed every 8 rounds, and what you've been doing on the second sleeve is every 9 rounds, which is why they don't match up and why you've got so much sleeve with so few decreases.
So you set the whole thing aside while you have a think about how you feel about the sleeves not matching, with the part of you that just wants to be done saying in soothing tones that at this gauge a few stitches' difference isn't going to result in drastically different sized sleeves, while the part of you that wants it done right is insisting that the sleeves should absolutely match, especially since this is a sweater for your sister.
And then you give yourself a shake, yank the needles, and rip back to the first decrease to reposition it, realize it was already positioned correctly, redo the round that really hadn't needed redoing, and carry on from there.
I think I'm back on track now.