For me, it's either
a) when I misjudge my damage and they survive my engage with an extremely low amount of hp
b) when I misjudge their engage on a bait and die
I only "tilt" when there's very clear things that I misjudged/messed up on, but by "tilt" I mean I get disappointed in myself and play safer for the rest of the game.
P.S.- While I was writing this I remembered the thing that truly tilts me:
c) As a jungler when:
1- All lanes are losing somewhat
2- Top and Bot are losing hard to the point that it could throw the game (it's difficult to manage both those lanes)
d) When laners "need" a gank by 5 minutes because they are 0/3 (that happens at my mmr) and complain the rest of the game that "jungle never helped". Like, what am I supposed to do if you messed up that badly before I even backed after my first jungle clear? xD
Rant Over~
Slythion