This is a situation I think most people have found themselves in before. It is unfortunate and quite unpleasant but at the same time quite comical if it happens to someone else and you are watching.
The ass face scenario goes like this. Normally this will occur on a plane.
Isn't it great when you finally get your food on the plane, the hostess or host has taken half an hour to get to you and you wolf down whatever it is they have prepared not caring that it tastes like fluff and then you sit back and relax and let digestion do it's thing. Now usually about 1/2 an hour later the one place you do not want to be now sitting is on an aisle seat for at least the first 7 rows back from the toilet.
This is why, you see people start lining up to use the facility and more often than not especially if it is a long haul flight this is not the first time that they have had to drop the kids off at the pool(Number 2's).
This previous evacuation of their nether regions has been allowed to stew and fester on a seat for say a good 6-8 hours and has now produced a torrid odour that can at it's most mild be called funky.
This person or persons are now lined up along the aisles either to reuse the bathroom or to stand up and get luggage. These people tend not to stand facing the front which precedes your first taste of ass-face. That is their ass in your face and I mean in your face and what can you do you can't run away. It locks onto your nose like a radar. These people aren't aware that they are even doing it or that their chute is emitting such a stench, you should see the looks on some passengers faces priceless and it's worse if you are still eating. This may happen to you on trains or buses as well if you are the seated passenger and it's not pretty.
Remember that next time you are travelling beware the ass face it will get you and it will get you good.