OK, I've learned the hard way. Under Tools, "Use Shift Key for Tool Switch" absolutely needs to be turned on. I do a lot of compositing, and I'm in the habit of hitting the E Key to get the simple Erasure tool, often to confirm I'm not on the Clone Tool. If you don't have this box checked in Preferences, and you forget that you already hit the E key once, it's going to toggle, and then wait to see what happens to your work. LOL. Thank goodness my History States are at 50. It took me a minute to see where I was screwing up, and going back in history made it an easy fix. I now know the box MUST be checked.