Mind games. Seeing a Nautilus Q coming right at you. Realizing that Udyr got sated at 13 minutes. There are plenty of things that scare people in League of Legends. Lets name a few.
Without a doubt, what scares me the most is when I'm bot lane support against a Thresh. Everything is going fine, until I see Thresh throw his lantern into that river bush that isn't warded and starts to charge forward. Less than a second later, here comes that Skarner on the lantern like a bat bullet out of hell.
Sadly, most of the time I see Thresh players do this as a mind trick. If the bush isn't warded, it always works to get my lane to back off.
Not nearly as intense, but I always get nervous whenever I hear that siren sound effect and know that Corki has his package ready. I always fear that he'll use it to hit some other lane that doesn't expect it.