What happens when you overmix Royal Icing?

A big issue many ccookiers seem to have is "overmixed" royal icing.
But, what IS "overmixed"?
And, what happens when you do this?
I decided to experiment with this concerning issue, to try and help other cookiers get to the bottom of this perplexing issue.

[@jamiebettis527]

Thank you for doing a test on this. I kept hearing not to over mix, but I've always mixed for about 5 minutes and never had a problem.

[@martierickson7937]

I made my icing too thin...arrggghhh, started over and rocked it!

[@Shedoesdiy]

Awesome! You can always just take the icing out of the bag, and put it into a bowl, then add more powdered sugar, then rebag. No need to start all the way over.