I'd say about 50-60% of the time when I start up a match in LoL my cursor gets locked into a tiny box in the bottom right corner of the game.
this is ingame on summoners rift, and I'm not the only person who has this problem. Sevral friends of mine have the problem of getting their cursor confined to about 1/4 the size of the games region in the bottom right corner. For some alt tab fixes it for others they have to QUIT the game and restart it to fix is (for my friend this means she's not back into game until about 3 min, because her comp loads slooooow)
Very annoying, putting that out there, hope you guys know about it.