I knew about four and I knew about them waaaaay after the fact (never saw a single one on my windshield but boy did I get the mailings 30 days later) but I was very polite and called the nice Parking Department man Sir (it is a good rule to be polite to state employees, especially ones who not only have to listen to Really Angry People all the livelong day, but can do magic things with a keyboard and some function keys as well) and he explained to me what the tickets were for and I promised it wouldn't happen again and he removed them from the record. Now it's off to the RMV to say "Hey, renew my registration so I can still drive, and I shall give you this fine ice cream cone. It is chocolate."
And just for the record: I've never heard of anybody else ever getting ticketed for having an old Cambridge parking permit on their car while legally garaged in Somerville, but there you go. (And the old permit's been removed since the beginning of this month so if we get any MORE phantom parking tickets, there will be tur'ble trouble.)