These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Although in the episode about France it was pointed out that the French have a very solid military record, refuting the Cheese-Eating Surrender Monkeys stereotype, and an episode on "Germany" instituted a special "Don't Mention the War" penalty (only for Stephen to go on to break it himself with a question about WWII).
Rob Brydon often takes umbrage at what he (jokingly) calls Stephen's offensive stereotypes with regard to the Welsh.
Alan being presented with a potato-head version of himself in "Corby".
At the end of "Justice", after a discussion about nude psychotherapy, all five panelists get out of their seats and get their lads out. Color bars appear and a continuity announcer says something's gone wrong. The show returns to the QI panelists back in their seats, joking about their relative sizes. Unlike other non-sequitur moments intended to segue into the next topic, the only thing left is the reading out of the scores (at least in the regular length version of the show).
In one episode, the audience laughed uncontrollably as the formula for paper folding comes up.
Stephen: What you need is length and thickness (to determine how many times you can fold a piece of paper).
This may qualify as a CMOF, as quite a few innuendos had been delivered by that point and Rob Brydon simply gave Stephen an innocent "Oh, really?" look...
Not to mention Doon Makichan absentmindedly miming eating a large deep-fried Curly Wurly. Two-handed. Linkski.
Harsher in Hindsight: There are many instances during the "J" series where Stephen Fry is obviously sick, on medication or otherwise not himself (the whole Jumpers episode being the most obvious example). Later that year, he attempted suicide (again), and in an interview afterwards talking about his bipolar disorder, he said, "There are times when I'm doing QI and I'm going, 'ha ha, yeah, yeah' and inside I'm going, 'I want to f*** die. I want to f*** die.'"
During the "Green" episode, Danny Baker urged the viewers at home to save electricity by turning off their television sets. That episode had one of the lowest audience figures of the programme's run.
When David Tennant was on the show jokes were made about Bill Bailey being on the Christmas special of Doctor Who. Come the 2011 Christmas special...
In early series, Alan Davies makes several jokes about being an oppressed and displaced Welsh person. And then, in the Europe episode, he reveals that he's found out that, despite his last name indicating otherwise, he actually has no Welsh ancestry.
In Series I aired in 2011, when discussing the fungus Cordyceps, Johnny Vegas speculates what kind of disaster it would be if it jumped into humans. Cue The Last of Us, a game featuring that exact premise just two years later.
In the Series F episode Future, Ben Miller's buzzer is the Doctor Who theme. Come 2014, and Ben Miller is playing the villain in the third episode of Series 8!
Most Annoying Sound: Sean Lock's crying baby buzzer in "Imbroglio". Sure, it was invoked (all the buzzers were purposefully annoying), but while the first two buzzers were annoying in an amusing way, Sean's crossed the line into this territory. Judging by the reactions of Sean (he doesn't often lose his smile, but he did for this!), Stephen, and the audience, they all seem to agree.
Nightmare Fuel: The occasional segment, but the "Inventive" episode was pretty horrifying for anyone with a fear of ventriloquists' dummies.