From the 70's a lighter version of the term; "Bad Scene," meant to be much less dramatic. It would rarely be used by the person who actually had the experience. Rather, it would be a show of support from one friend to another, though it could also be an attempt to trivialize the experience.
In extreme circumstances, it could also describe an ugly experience with drugs, violence or sex.
Bummer is a shortened version of bum scene.
The word 'bum' would often be said while holding in a hit of weed followed by the word 'scene' that would be stretched out while exhaling the hit. As in: Bum (holding breath) sceeeeeene (exhaling slowly).
#1: I lost my wallet with 20 bucks in it.
#2: Bum scene, man.
#1: I was so messed up I think I left it at your girlfriend's house.
#2: Bum (holding breath) sceeeeeene (exhaling slowly).