Airspace violation
I know of a training flight that got into class B without clearance. The instructor and student had just departed an airport and then got flight following, but in the time between the tower and approach frequency change got into the Bravo. The controller tersely "suggested" they leave the bravo. Nothing of violation or phone numbers to call over the radio was mentioned.
Are they in the clear? Or does the controller use the tail number and the FAA N number registry to look up folks and send them a letter? I have heard the tower ask for people to call them and I have heard the words pilot deviation over the radio before. In this case neither occurred- but are they in the clear? A NASA report was filed.