Situation where you didn't intend to : You have just taken a SC in the Spring, and you see an open SC to the North. You have two armies adjacent to your newly acquired SC, but none adjacent to the open SC to the North. In a gamble for two builds, you move your army out of your newly acquired SC into the open SC, while supporting one of your armies in to secure your Spring SC. Your enemy bounces you at the open SC. Voila, you have knocked your own army out of the Spring SC.
Situation where you exploit the bug: Your fleet is adjacent to two SC's, one is empty, the other occupied. No other hostile units are nearby, and you have two fleets in adjacent seas but are not adjacent to the two SC's. Your opponent may choose to move his army into the other SC, or hold in the former, giving you a 50-50 chance of capturing one of the two SC's. Instead of taking a 50-50 chance, you dislodge your own fleet with a supported move from your other two fleets, and during the retreat phase, you simply waltz into whichever of the two SC's is open, regardless of your opponent's move.