I'll never forget one afternoon in the early '70s, when I was watching a program on Cleveland's NBC station, WKYC-TV. Someone in the master control room must have flipped the wrong switch or pushed the wrong button on a control panel because, to my amazement, during a commercial break I saw the ID for New York's WNBC! First time I ever saw a slip-up like that in the Cleveland area (I lived in an eastern suburb of the city at the time). Did this sort of thing happen anywhere else in this country besides Washington, D.C. (as a previous poster, W. B. I think, noted) or other major TV markets?
I wish I'd had a VTR at the time (VCRs had not yet gone mainstream in this country in the '70s because they were so new and so expensive) to capture this gaffe for posterity. I have not seen another one like it since, although with so many TV stations and networks, such as MeTV, being automated these days, I wouldn't be surprised if it would happen again -- entirely by accident. I have seen snippets of commercials on MeTV during commercial breaks, but I have yet to see slipups like this on the major network affiliates, since the one I just mentioned.
BTW, I also remember one hilarious slipup on WKYC-TV one morning in the early 1970s. The station was minutes away from signing on, but instead of a test pattern, I saw, to my surprise, a station ID that started spinning, a "BOING!" sound in the audio, and I don't remember what else, but it was a hoot to watch. An engineer must have spotted this long about now and switched to a test pattern.