You mess up stockinette stitch. It might be almost time to turn the first heel of the first sock. I couldn't wait, so tried it last night. The second time I got a garter ridge, it was time to quit. Not being able to remember how far across the row to knit should have been my first clue. I'll probably end up ripping out the heel and knitting a few more rounds, then heel-turning. (Kind of like ankle-watching.) I may have started the heel too soon. I think the heel is of the short-row variety (e.g. it should look like the heel of COTS socks.)
I can never get the first heel done on the first try....maybe one day.