Hello! Besides a problem with the disappearing cursor of a mouse there is still a problem - function of active corners of the screen sometimes ceases to work. After the cursor you move in one of corners - occurs nothing.
Because of these two problems I very much regret that passed with Lion to Mountain Lion
If you open Activity Monitor and kill the dock this will restart the dock which also controls hot corners. If this fixes your issue then you may have to delete the apple.dock.plist in your Library. This will reset the dock so you will have to set it back up, but it should help your problem.