This one really took me surprise when I first saw it, especially given that I was in the flat on my own at the time and couldn't shout anyone to the TV to make sure I wasn't going completely mad.
While Metal Gear is probably the ultimate series for mysterious happenings, cheeky easter eggs and bizarre fourth wall breaking occurrences, this moment in MGS3 is quite difficult to come across. This one happens if you save and reload your game at a very particular point (just after an autosave, funnily enough) making it a slightly unsettling experience if you're not expecting it:
http://www.youtube.com/watch?v=Grd8JBMusDk