That usually happens to me when I'm sewing when I'm tired and fog headed. It used to be that was late at night, so I called it the After Midnight Curse. I've done some spectacular mess ups after midnight. These days, I generally quit for the night at 9, and I still have made some frustrating mistakes.
My latest episode of twitter patedness was to copy the templates for a block from a book, only to fail to recognize I'd moved off of the 12" block pages, and on to a 9" block page...
Felt fairly stupid about that. THEN I realized that not all the templates were correctly sized anyway. I'd copied them correctly, but once piece just wasn't right at ALL. I quit and went to bed. Got up the next day and got out my grids and drafted the stupid thing from scratch! It's a fairly annoying book anyway, and has NO assembly instructions for any block, so you're on your own. Which isn't always best for me!