To continue on with my last post, let me recall an encounter with a clay golem. The PCs were not that well equipped to deal with it. One of the PCs thought that, perhaps, having her summoned earth elemental Earth Glide into the golem, then try to possess it, would work.
Now I seem to recall that golems had an elemental nature back in AD&D. So during the game I said, sure, since it's insane (lost control), you can replace its elemental spirit with your summoned creature - if the clay golem fails its save. So I had the PCs tell me what 10 + CON mod + 1/2 HD was, then rolled a Will save against that.
Golem failed, the PCs killed it, nice work. Pretty cool moment. Totally ignored the rules.
(Which is why I am a fan of 4E - page 42 would handle that nicely. Atk vs. Will, high limited damage.)