Ohhhh where do I begin.
Sewing two layers instead of one--uep done that.
Mis-cutting pieces--oh that was a costly one for me. It was for a wedding quilt and the largest piece in the block. Thank goodness I only cut half the pieces needed so only had to replace half the fabric--plus shipping of course.
Sewing the wrong side of the fabric--how about sewing a whole block into a quilt inside out and not noticing until the top was put together. Of course it wasn't an edge block, was it.
And my favourite--marking the line for joining the binding and then cutting on it instead of adding 1/2 inch...that earned a few bad words.