A forfeit in QI is given when an obvious, incorrect answer is given, where the back screen will turn black with white flashing text of what the person said, the lights will flash different colours, and a siren will sound pointing out the forfeit. Especially in later series, forfeits are sometimes obvious joke answers and occasionally even obvious answers that are technically correct.
The forfeit is sometimes described by panellists as a "klaxon": in "Knees & Knockers", one of the questions addresses this. It is not a klaxon, as this is a specific brand name owned by Lovell-McConnell Manufacturing; "siren" was suggested as an alternative description.
Most forfeits deprive the contestant who caused it of 10 points. Some forfeits can take away more points: for instance, in "Atoms", the forfeit "carbon dioxide" would have scored -3000 points, although nobody said it.
In later series, the forfeit will occasionally talk to the panellists. For example In “Plants” the forfeit tells Alan Davies to not eat the soil, blaring: “NO, ALAN! DON’T EAT THE SOIL!”
Forfeits in the Pilot did not trigger a light change or klaxon; the incorrect answer was written on a card held up by Stephen Fry whenever the answer was given. In earlier series, incorrect answers were displayed on both the screen and on pre-printed cards; however, the cards were dropped in later series, and the screens remained as the only indication of a forfeit.
- In "Fingers and Fumbs", a forfeit is given whenever a panelist says "a particular 'f-word'" during the episode. When the word is said, the regular forfeit klaxon is replaced with a screeching sound, and 'F*#@' is flashed up on the screen. Additionally, saying the word triggers a game of paper-scissors-stone between Stephen and the offending panelist; if the panelist wins the round, the forfeit is halved to five points, and if Stephen wins, the forfeit is doubled to twenty.
- In "Germany", a forfeit is given whenever a panelist mentions "The War" - i.e. brings up World War II or WWII-related topics.
- In "Hodge Podge", Alan Davies' buzzer was a forfeit. It was only rung once in the episode, at the beginning, and when rung, the klaxons were sounded and 'Minus 10' appeared on the screens.
At various points in time, the record for the largest number of forfeits triggered within a single episode has been: