Sometimes the sights of the weapon (this problem only applies to reddots, holos and ironsights) are displayed at the bottom of the screen, making it very hard to use the sights in question.
This occurs quite randomly and I am not able to reproduce this bug. Also, as far as I know, there is no way to get rid of this bug apart from restarting the game.
Screenshot: http://i.imgur.com/IqjwsN7.jpg