I know that monsters have programmed gambits of their own, and that sometimes leads to weird happenings. While farming for Moondust from Mus in the Feywood I stumbled across a strange behavioral quirk in the Mirrorknights.
To save time I fled from every Mirrorknight encounter until I reached a Mu, usually in the clearing near the Henne Mines entrance. Their first action when I got in range would be to cast Reflect on themselves. Then they would take off in pursuit. By the time I stopped fleeing, two or three Mirrorknights would still be running after me and usually they'd just keep on running past me! It was like they were out jogging or something!
Has any monster done something strange in your game?