Let me just say that I have done the almost exact thing as you, a couple times. I have been sewing for 65 years and making bias binding for probably 50 of those years. And the times I have screwed up sewing binding together have been in the past year. I think we try to hurry, or get tired, or get it in our mind that these are simple tasks and just don't pay attention to what we are doing. Just relax and take a little break.