Let's see...
Not paying attention to which rectangle a corner square was supposed to be sewn to, and sewing 40 of them. Undone, then sewed on the correct rectangle but on the wrong corner, on all 40. Then repeated the first mistake with a second set of 40 cut pairs. And this was with labels pinned to each stack of cut pieces - I just simply looked at the picture and didn't look at the piece labels in the instructions.
Stitching on a diagonal line and cutting 1/4" off, when I was supposed to stitch 1/4" on either side of the line and cut on the line. At least I caught that one early while I still had more fabric (the excess waste on the first few is what made me stop and check).
Not realizing my spool of monopoly had partly unwound and was hanging out of the drawer and on the floor. I vacuumed up about 1/4 of the spool before I realized what was going on. No way I was going to try and recover all that lost thread.