Re-littering

Where do you all stand on the subject of re-littering? I.e. if you see something on the street that you go 'Oooh - that seems interesting' over and then you pick that something up only to realize that it is just a piece of crap - is it then ok to throw it back down on the street or should you locate a trash can? It happens to me quite a lot and I often get stuck in this ethical dilemma (most oftenly I do re-litter but I never feel good about it).