Review the rationale about mistakes being okay if you learn from them. That is why pencils have erasers. That is why we have word processing programs for computers with delete buttons. That is why we have U turns. The neat thing about making a mistake is the learning that you can gain from it.
Mistakes are for learning.
If we are smart, we don t have to keep making the same mistakes over and over like the one trick pony. Recognize your errors with the cheery message that mistakes are allowed in your class as long as people learn from them. This gives a positive model to the children about learning through errors. Help the child to see that blaming someone else is an unnecessary defense. That doesn't help solve the problem.