Guacamole starts to smell bad because it is spoiled, and you should throw it away. Also, if guacamole has a strange taste, don't eat it and discard it. If your guacamole got some lint or black or white spots, it developed mold, meaning it has been wholly contaminated, and you should get rid of all the sauce, even if it has only come out in a small area.  If guacamole has a strange appearance, whether very brown or watery, check its smell and taste before throwing it away.