One of the worst things to happen to a champ in your champ pool is for the champion to get noticed.
One of the biggest exampes would have to be Kench.
Kench was one of the lowest winrate supports before any of his "big" changes, and for pretty decent reason: He lacked in poke, solid engage, sustain, etc, very much a jack of all trades. However, players (myself included) loved to play him support for the sick devour plays, where you could flash consume an ally from certain death, causing cheers from your team... or eating an ally and then flashing over a wall to save you both. It was a great feeling that felt rewarding.
Then Riot and LCS noticed. :C
Riot wanted to change him to a top lane juggernaut to try and stop his safe playstyle as a support. I played my fair share of kench top, so while I was immensely bummed that kench was gutted as a support, I accepted him as a toplaner. As overtuned as he was, he was amazingly fun toplane....then he got hit with a justified, yet drastically harsh nerf hammer. Now kench is utterly useless to anyone except the .01% of players.
Another example is pyke. Pyke when released, in all honesty, was underpowered. He felt weird to play, and players struggled to play him. He was adjusted, and players saw that he was fun. Ironically, it was after LCS and after he got some drastic nerfs (from the aftershock adjustments to his waveclear to his tank builds) that people began to complain about him. I still don't think he's a problem, and that hes in a ok spot as a support right now. I kinda feel bad for the OG pyke mains, as their champ is probably gonna get nerfed again because the majority of people refuse to change their playstyle when fighting him and as a result, feed when fighting him.
Then theres aatrox. I really don't know what to say to any aatrox main, be it prerework aatrox or post rework aatrox, except damn. Im sorry for how they massacred your boy.
A few games ago, I played illaoi into an ignite riven main. She refused to play differently around my champion, and as a result, called my champion "c@ncer". I would go something like 1/7 that game, with riven going 10/3, and yet she called me "unfair to fight against to any melee champion". I guarantee you, if illaoi was immensely popular for some unbeknownst reason, people would only see her positives and refuse to play differently/ abuse her weaknesses. Example sample complaints: "OMG her burst is so stupid and shes so tanky" "Wtf is that healing? Why should a tank heal that much" "0 counter play c@ncer champion gg" "Braindead af"
Its easier to complain like that then to learn to back up or sidestep, isn't it? Thank goodness illaoi is one of the least played champions in the game, because lord knows if she were noticed by others or by the LCS she would lose all identity that made her a niche to pick in the first place.