When you're super-fed, a team fight starts, your teammates all die almost instantly...
And the enemy team decides to try and take you 5 on 1:
And the enemy team decides to try and take you 5 on 1:
How I feel when I'm 
Name one champion that won't get screwed over if he tries to take 5 people at once? Illaoi might kill maybe 3 but not 5. Come on.
how i feel when i play
and i have
at 30 minutes...
Are you SURE you want to fight me?
that feeling when u were playing old full crit yi and has PD+IE+SS+BT+attackspped boot+GA in 25 minute
when he is ganked and you are fighting in his turrets.
Can you give an example of a champion against a legitimate team comp who could actually 1v5 a fight without the enemies making huge misplays, even if he's ahead say 15/3/1?
I mean, I see it happen (what I'm saying is it's not because the 5v1 was a bad fight, rather because the 5 misplayed terribly). WAY more often though, I see said fed player get 1 or 2 kills before dying (sometimes 3, very rarely 4).
Aside from them running at you one at a time, or all being down summoners and ultimates and even a few regular cooldowns... I just don't see it. I guess a reset champion could pull it off, but even then a little hard CC would shut them down pretty quick in a 1v5.
I was on the other side of this one just yesterday. A tank Akali (how is that even possible) ran over our team a couple of times. Bronze problems, I know, but man, did it suck...

Full AD
that ults the entire enemy team together
What movie is this from?
This reminds me of a Twitch I fought some time ago. We had just won a teamfight and where doing dragon. All five of us. The Twitch wasn't even very fed, but as we where doing dragon, he unlstealthed, and unleashed his ultimate. Because is is a penetrating AA, he did a massive amount of damage to all of us, and we died. He got a Pentakill.