wow, you have LEFTOVER avocados? ;-)
i've heard a few things... that if you leave the seed in, it's less likely to brown. also the browning can be stopped with a little lemon juice. it certainly doesn't hurt the avo, it's like an apple... it's the air that makes the browning happen, it's not mold or anything.
i've eaten plenty of brownish avos and they were delish. if you make guacamole, you've gotta remember the lemon juice or it turns all brown fast.