Steps to reproduce:
1) Add a new texture to the world
2) Set the preview selector (Texture/World/Both) to "Both" mode
3) Adjust settings of the texture, and watch as the usercount of the texture increases everytime a new preview render takes place to update the texture preview

Saving and reloading the file resets the usercounts appropriately, but starting to tweak the texture again will start increasing the usercounts again.

Tested on r.51273, Blender Internal.