Version: 2.71 official (linux 64)
How to reproduce:
# Open the attached file and inspect the simple "watch the cube spin" game.
# In the action editor, unlink the CubeAction.
# Notice that CubeAction is now listed as having 0 users even though it is clearly still being used by the actuator in the game logic window.
# If you save and re-open the file, the CubeAction will be cleaned up, leaving an orphaned logic brick and breaking the game.
It is implicit that the Action Actuator should count as a user of an Action datablock when other things like action constraints count as users. Otherwise, the game artist must add a fake user for each one of the many actions that a single object/character could perform in a game. Forgetting to do this once can result in lost work.