I have had many a day like that! Sometimes the obvious is just not obvious. For some reason our brains know what we thought we did, thus skip checking that what we actually did was correct. I spent several days like that while trying to quilt a quilt. Can't tell you how many stitches I took out because of the mess that showed up on the back of the quilt. Finally went to the machine to check it one more time before taking it to the shop...and saw that the machine was not threaded correctly. I only missed one, tiny step when I threaded it, but, oh, the extra work that missed step caused me!
It's not that we are dumb. We're just imperfect humans.