My biggest moment was while making a traditionally pieced queen sized storm at sea. I like to do all of a step at a time for consistency, and I sewed all the skinny triangles to the large diamonds wrong... took them off carefully, went to a fresh side of the diamond and sewed them on all wrong again. I learned some things. Test blocks good... angles work this way... I didn't like undoing then and I hate it now... The rest of you might have learned some new words and phrases from me!
That quilt hung in a large guild show. It wasn't until it was hanging in the show that I realized one of the pieces was wrong side out -- it sort of did make a difference but wasn't too noticeable in a scrap quilt. I had handled each block something like 27 times while making them and assembling the quilt -- and then quilting it all down and up until it was hanging I hadn't noticed the inside-out fabric. I say that fabric has two sides and we get to choose which one we have up, but I had chosen the other side.
Oh well, it was a gift and well received and appreciated and so far as I know they never noticed the one block. At least nothing was said.