I took off my rear derailleur to clean it, and when I went to put it back on the threads were too damaged to get it back on. I've never had this happen before; anyone else? Can I get it re-tapped? Will it be as good as new? I'm nervous that it's been damaged by road salt, and lost metal. It's a nice steel bike that's only about three years old, but I've ridden it every day through Minnesota winters.

you may be cross threading it and have damaged the threads ..

yea a bike shop should have taps to chase the threads and straighten them out

^^^THIS^^^

I don't see a major problem developing.

Best of luck to you.

Did you apply grease to the bolt before attempting to reinstall? If not, always do that, especially if the threads were probably already a bit rough with corrosion. And like said above, most likely the threads just started to get a bit cross threaded and should be easily fixable by any competent bike shop.

You can try to chase the threads by installing the derailleur mounting bolt from the back side of the hanger. It should thread all the way through and fix any damaged or crossed threads. Clean up the threads well, and apply liberal amounts of oil (cutting oil, ideally, but standard oil is better than nothing). Use a cheap spare derailleur mounting bolt if you have one.

This ^
Not removing any metal this way similar to a roll form tap, a superior tapping method in almost every circumstance.