I had a (ranked) game today where I was completely unable to see 2 of the enemies ingame, Hecarim and Rengar. Not on the minimap (not even as blue squares like missing icons sometimes show, just completely invisible), not on the actual screen (as if they were invisible - but not showing with true sight either), not on score screen (tab); when they killed someone a placeholdertext was displayed, when they chatted in all chat it was green without championname. Their kills didnt even count in the sums, though I could see the correct death-count on my teammates in scorescreen (it counted as if they got executed). I killed them once by aoe damage, it didnt update my kill-counter (dont know if i got gold, but i think I got it), if i got killed it increased death counter (like execute); so the team-score, my score on the upper right corner and the score in tab basically displayed the score the game had without Hecarim and Rengar in it (both death and kills).
They could kill me, death recap displayed placeholders as well. I could see the effect they made (eg, one of the players was Rengar, I could see the "!" on me) but not the actual champion - and no, I dont mean he was invisible for a moment, he, and Hecarim, were invsible the whole game for me, I basically just took damage out of nowhere, couldn't aim at them, but aoe damage was possible (killed them once). The rest of the team could see them.
It was like my client didnt know they were there, but got data from the server about the effects.
Happened just once by now, no idea how to reproduce it.
And now some screenshots to show what I mean (I removed the names).
Scorescreen: 2 enemies completely missing (yes, it was a 5v5 ranked); the score 6-12 represents just the kills and deaths of the other 8 champs, although you can see, Heimer died 10 times (to Rengar and Heca), its not included in the total score (it is displayed as if he got executed, but he got killed).
Death Recap, just placeholders:
I asked Heca and Rengar to say something in all-chat; they talked in green without champion name. It's a shutdown too on Rengar, the icon isn't displayed (just blue square), and the textchat says: "xxx (Lee Sin) has claimed Unknown (Unknown)'s bounty! (Team Gold: UnknownG)"
The aftergame stats were displayed correctly.