Inspired by this thread:
I was thinking of the opposite. I've been on a quest to replicate Carol's cookies which are tall (a few threads on them here). Recently, my daughter made ccc cookies recently and left out the brown sugar. We baked them anyway. They were great--nice, tall, not too sweet but sweeter than a scone w/ the same height. It helped that she accidently used milk chocolate chips. So, I'm using that as a springboard to perfect the tall cookie.
So, what mistakes have you made that opened a whole new door for you?