Being יוצא על המחיה from someone else
This is a prevalent minhag in camps and other places. After eating mezonos, one person gets up and says על המחיה and everyone else is יוצא by listening and answering אמן.
The problem is that this is against the din. We see in todays daf (Berachos 45) that שנים שאכלו כאחת מצוה לחלק, two people who ate together should split up and each one bench for themselves. We see from the gemara and the rishonim that whenever there is no zimun then each person should bench for themselves and not be יוצא from someone else. Since there is no zimun by על המחיה it is the equivalent of שנים שאכלו כאחת even if it is 100 people and מצוה לחלק, that every person should say the beracha himself.
The best thing to do is have 1 person say על המחיה and have in mind to be מוציא all of those who don't know how to say על המחיה and anyone who has a safek (maybe he didn't eat a shiur), and everyone else should say על המחיה word for word with the person so that they make their own beracha.