Yeah, it's a shame when a container is abandoned and essentially becomes trash -- in my experience, it's rare for that to happen, though.

On the other hand, there's 'Cache In Trash Out' events (CITOs): Geocaching events where people gather to pick up litter, remove invasive species, plant trees, or otherwise help the environment. They're pretty popluar in my local area. So I wouldn't say geocaching has a net negative impact as far as littering goes!