The level 25 talent for Death Prophet grants hasted movement during Exorcism.

However this haste does not end until all of Exorcism's spirits have returned to Death Prophet.
Since the spirits have a movement speed of 500 and haste grants the hero 550, you can actually keep moving away from the spirits to retain the hasted movement.

I'm guessing this is very nit-picky since there is an unlikely situation where this would happen. I just thought it would be strange to have a potentially permanent haste.