My first quilt was to be 12" blocks. Each of the 20 was anywhere from 11 to 13. I was teaching myself from a beginner quilter's book. I had no teacher, but decided to frame each block in a contrasting color. I applied 2" strips to all four sides of each block then trimmed them all to make 14.5" blocks. When looking at the quilt afterward, the difference does not show. The points were crisp and blocks were framed with .75 - 1.75" frames. The quilt grew quite a bit, but looked great. I used this quilt in a demo at my guild and members were surprised that such a mistake was even there.

I've also read this is a myth--no built in mistakes.