Tape Switch

The programming for a television station is stored in a large "refrigerator" containing data, video tapes and other media. Tape Switch occurs when it's 4 p.m. and Tony in the broadcasting booth accidentally puts in the hair gel commercial instead of the Gillette razor commercial that's supposed to be in there at 4 p.m., and you get a "blip" of the wrong commercial, or maybe half a commercial, then a complete commercial for something else. The same "blip" occurs nowadays for a different reason with some cable companies, who overlay their own local commercials in some breaks; the blip happens if Tony at the cable company is a second too early or late in flipping the switch.

This also happens in radio. When it cuts to the same commercial ("Hey, Tony, I guess it was the right tape after all!"), it's Ad Nauseam. Often the commercial they yanked may be more intriguing than the one they put in, especially when it's the local cable operator cutting away from the network feed for local commercials (usually to advertise the service you are already watching it on).

Examples:
 * The Sci Fi Channel (now called SyFy) frequently made this mistake with a certain video game production school. It would show one commercial, then two or three seconds of a creepy CGI skeleton guy.
 * At random times during the day, the Bott Radio Network in Kansas City will have the announcer say "SIN", and then cut to a different commercial. I still don't know what commercial that's supposed to be.
 * It's not a commercial, it's a command, so, you know, start sinning.
 * During LOST's 4th season, this Troper's LOST group would regularly see a local commercial touting social security (we think), just before the end of the commercial break. It was always just before the show resumed, and only the last second of the commercial was shown, making the word 'security' muttered by an aging woman the unofficial prompt to 'find your seats, the show is about to begin'.
 * A variant of this has cropped up in modern cable television: About every half-hour on cable stations, the channel's commercials are pre-empted by the cable company's commercials. If you're lucky, they'll sync up correctly. A little less lucky, and the company's commercials will end early so that you can see the tail end of the channel's commercials. Even less lucky than that is when the company's commercials end late...
 * This is unintentionally subverted on the National Geographic Channel as broadcast in Israel, where it is broadcast on cable based on a feed from other European countries. Most of the time, the cable trailers completely overlap the commercials, and if they don't the cable company broadcasts some local wildlife footage to make up for the gap. Many times however, they forget to pre-empt the commercials entirely. Many viewers of the channel (few as they are) actually enjoy this, because it gives Israeli viewers a chance to see actual commercials from other European countries, thus inadvertently fulfilling National Geographic's "cultural-exploration" theme! It helps that these commercials are often in Dutch, Romanian, Turkish, etcetera.
 * Sirius' Classic Vinyl station once started playing a few notes of a song...then silence, an "Oops" from the DJ, followed by a completely different song. Really curious what happened there.
 * Noticed on GSN. It is always a commercial that starts with a teenager shouting "You and your rules are stupid!" and then promptly cuts to a different commercial. I've officially only seen the full commercial twice, and the tape switch too many times to count.
 * The juxtaposition of the ads makes the teen's comment rather interesting in context.
 * ABC Family is a big offender, especially when you're watching shows like The Secret Life of the American Teenager. It can go from "before ad ends, insert local ad, end with last 2 seconds of the original ad", "original ad starts, interrupt a second later with local ad, end late, rest of the next original ad continues with a first few seconds missing of the ad", or a combo of both. And it doesn't get spared at all, no matter what pay-tv provider you have, it will happen.
 * Nickelodeon sometimes have this by a milisecond.
 * There's a few rotating network promos just before the last segment on NBC Nightly News; at least one local station runs a brief weather update over it.
 * Many years back, a hilarious example from a UK commercial station had them running a promo for a steamy drama called Bouquet of Barbed Wire. As the actors chewed the scenary, unfortunately the soundtrack came from The Muppet Show and Kermit the Frog.
 * This kind of mistake is actually very rare on UK television, thanks to automated time cues.