I was trying to make some terms exact match and when I hit save I got this message.
I get it on average of once a month. I thought it may have been because I had another session open either in a different window or on another PC.
The multiple AdWords windows are a usual habit of mine though, so I can only imagine the error is caused by a stochastically determined set of circumstances ;)
I just move on (and always paste to Notepad first!)
Israel
Advertisers wouldn't have to clear the cache and cookies so often if their programming was better. Fix the problems, don't just teach the 'workaround'.