Metagaming examples that seem too mild to call cheating, yet make the game less fun:
"We're playing a Living Module and the XP budget only allows so much, so that dragon cannot be real. I ignore it and attack the other guy"
"Although my character has no knowledge of aboleths, I do, so I stay at the back and let the other players charge it"
"I read about Tomb of Horrors online, so I'll let the others climb into that mouth and not die myself"
"The Gm knows that the young kid playing the druid will get super-upset when their animal companion gets hurt, so we'll use it to scout ahead"
etc., etc.
In all cases, these are often unvoiced thoughts. I've seen them ALL as a player and as a DM (an equivalent for the ToH example) and although not worth hate, per se, extreme dislike is warranted.