If you're a mage, and an opposing mage casts polymorph on you, wait until the last possible moment before he sheeps and hit mirror image. If done correctly, due to latency, you will summon your mirror images after you get sheeped, making them take the form of the current polymorph effect on you.
Example:
This can be quite amazing for duels and arena if the opposing mage goes for a sheep->evo combo, your mirror images will attack him while you're sheeped and push back his evo or whatever other spell he tries to cast, not to mention the "WTF" factor too.
Summarize:
1. Have another mage cast polymorph on you
2. Wait until the last possible moment before the sheep goes off and hit mirror image
3. Your mirror images will be summoned and will take the shape of whatever polymorph is on you!