Alright, alright, I'll put you all out of your misery looking for mistakes!
I suppose it's not really a mistake, more something I should have done but neglected to do so.... I cast on and knit the rib with 3.5mm needles and then changed to 4mm for the pattern part. I should have changed back to the smaller needles for the second rib but I forgot to. If you look at the ribs you can quite clearly see that one is wider than the other, but you won't see it when it's blocked. And besides, they're wrist warmers, which means one end will be tucked inside a coat or a sweater or something!
Yeah I think I'll be wearing them soon without waiting until the fall. I head off to England in a couple of weeks and they've had a lot of snow and the long range forecast is "unseasonably cold for April". So I'll definitely be taking them with me!