Curious thing but thought I'd pass it along. It applies to Airbus but I can't say if it's the same for Boeings. In the aircraft (not the sim) the lenses on the O2 masks have a protective plastic film over them to protect against scratches when they're stowed in the box. It's easy to not notice them in the heat of the moment (sorry about the pun) but if you don't remove the film when putting your mask on, they can do a nice job of obscuing your vision in an already difficult situation.