I started getting this problem on my Late 2013 13" rMBP. I'm not sure when it started but I'm on 10.10.3 right now. Only way to fix it is change hot corners in settings or killall dock in terminal. The weird thing is it happens randomly and if I ignore it, it'll fix itself and then come back again randomly. Even weirder is only top right, bottom left and bottom right corners stop. My top left corner never stops working. I tried changing the actions of the hot corners around but it was the same corner that kept working when others stopped. My dock is on the left in case that matters. Anyone have a permanent fix?