i know that when you are first outlining a blank cookie with RI, if you make a mistake you can just wait til it dries and then scrape it off. but what do you do if you have already done a base layer of icing, and you've made a mistake detailing on top of it?

Sometimes you can pick off the detailing if it's quite firm to begin with, but if it's spread much, you can't because it would damage the base cost too much.

I have scraped off ALL the icing before, and started again, when I realised something was wrong and it wasn't going to dry (grease in this icing).

Those would be the ones my DH gets to eat!

Yup, my kids also trawl the kitchen for 'screw-ups'!