I am Low ELO. Apparently, every one of my games has an Autofilled individual. I queue up as Jungle/Fill/Bottom. If our team didn't have one, the other team did. If the individual makes a lot more mistakes and does not optimally use their champion at any point, then they're typically an Autofill.
When I played a lot of ADC, my support was often Autofilled, so they didn't do a lot of support things they needed to for us to secure a win. Even if we demolished lane, they'd overextend and pretend to be the AP Carry anyways...just to die a gruesome death and give us a 4v5 situation. If they weren't an Autofill, then they'd at least try and stay close to me regularly during fights.
I've noticed more Autofills will typically flame and provoke negative behavior. A lot of the time, I've seen them basically say "whatever, this isnt even my role choice" and play haphazardly for the rest of the game.