Farfel the dog, the ventriloquist dummy mascot for Nestlé back in the 1950s and '60s, would end his familiar jingle (N-E-S-T-L-E-S/Nestlé's makes the very best/chaaaw-klit) with a "chomp" sound. This was because Jimmy Nelson — who performed him — was nervous during the auditions, causing his sweaty hand to slip off the mouth control. The executives liked this so much they they kept it in ever since, even when Farfel was revived (along with his extended family) in the '90s for a Christmas commercial.
Parodied in a Geico commercial where Foghorn Leghorn is doing narration for a book, only he is doing a very bad job at it. At one point in the shooting, he also adds in something, which he shortly thereafter admits that he added it in. Eventually, it got so bad that Henery Hawk ended up departing from the equipment room with a bat and was audibly implied to have bludgeoned Foghorn Leghorn with it, presumably to beat some sense into him.
From a famous Wendy's commercial: Clara Peller was not wearing her hearing aid when she said her classic line, "Where's the beef?", so it came out as a bellow. The producers thought it was funnier that way.
"Does Exactly What It Says on the Tin" was a placeholder for a proper marketing slogan in a series of adverts intended to show that using a woodstain is simply common sense. Eventually the advertisers decided to go ahead and try it as an anti-slogan. The rest is history.
This was referenced in a later commercial for the Da Vinci Pro Cutting Tool, where Vince throws a $200 food slicer out the window.
When Tom Bodett was chosen to record advertising for Motel 6 in 1986, he ad-libbed the line "we'll leave the light on for you" at the end of the commercial. Nearly 30 years later, he still says that line in almost every ad.