I only encountered this a few times, but I am mentioning it anyway. Your agent will keep walking while he remains on the same spot. (Just like Michael Jackson.) Here are hints you can try to get you out of this situation:
1. Choose another agent and click on a different location.
2. Let the agent use a weapon.
3. If you have used the red or blue drug, disable it.
4. No psycho gas around? (No bug intended.)