Interestingly, I experienced Newton's Rings for the first time in my last darkroom session. I was printing a TMax 400 negative in the Leitz V35 enlarger, and it was the last negative of the particular strip of film, and more curly than usual. So I decided to use AN glass on the top to hold the negative flatter; I could visibly see it curl as it was in the neg carrier.

I got what I thought was a nice print, on a piece of 8x10 Ilford paper. Fixed it, toned it, dried it, flattened it, and scanned it. Lo and behold - Newton's Rings in the upper left hand corner. That had never happened to me before, and now I wonder how I'm going to print this nice negative without having issues...
Example attached.