It does seem to be a bug, and the "Ghostman" scenario as described earlier explains it fairly well.
When you have a situation where you say "I will go here and do xxxx" your action is already considered spent. You begin moving, reach the position, and do the action which you paid for already.
If something happens, from AoO, to a damaging/difficult Terrain (you will sometimes stop as you enter these) your movement stops and you only spend the appropriate amount (You planned to run for all of your movement, but only reached halfway you'll have half your Movement left when "Stopped"), but your action is considered spent, without actually doing the action.