Every now and then while watching tv I see a second of another commercial between two others. What is the purpose of this? Is this just like a backup in case whatever mechanism is showing the commercials fails there will always be something on the screen? Is it just a software issue that no one cares enough about to fix?
What you’re probably seeing is a “local availability” commercial. The networks and syndicated shows generally run their own commercials, but they also give the local stations a couple of minutes each hour to sell. During that segment, the network or syndicator will run a public service announcement, promotional spot or a “pre-emptable” commercial to fill the space.
Since it’s next to impossible that the local commercial is exactly in sync with the filler material, you’ll usually see a glimpse of the filler material at the beginning or end of the commercial, or else a blank screen for a second or two.
Do you have cable? You’re probably seeing a slight timing-mismatch when the cable feed switches from a local ad to the underlying feed from the network. If the switch happens a second early, you’ll catch the last second of whatever ad the network was broadcasting at the same time.