[sigmaprimus]

Object recognition only and using a phone camera limits my ideas,but off the top of my head:

Actors in TV and Internet advertisements

Animal scat or Animal footprints

What the stains on someones clothes were made from (yuck)

How many jelly beans or whatever are in a jar

The freshness of produce (especially those pre-made salads that can wilt before you get them home)

Poisonous plants and mushrooms