In Diplomacy, the one "special case" rule revolves around the Convoy Paradox. Simply put:

If an army attacks via convoy a province whose army is supporting the convoying fleet, and the convoying fleet would be dislodged but for that army's support, what happens?

Answer: Nothing.

If the fleet were dislodged, then it cannot convoy the army which cuts its own support, so it would not have been dislodged.

If the defending army is dislodged (the convoyed army had support), then the convoy would be dislodged, preventing it from cutting its own support.