The cowbell of shame is used when one person on the show makes a joke that bombs. It originated when Patrick and Adam got their Wacky Radio equipment, but stuck out from the slide whistle, horn, and others and is now the only one still in regular use.

Circa Episode 250, the "mini" cowbells became available in the store.
Ironically enough, the cowbells were anything but mini, and were actually larger than the cowbell used in the studio at the time. After a small debate, it was decided the new cowbell would be used from then on.

If you are extremely bored, you can distinguish the two cowbells by the pitch of the sound they make. If it is low, it's the new bell. If it's high, it's the old bell. This requires you to be a loser, so keep that in mind before attempting this experiment.