Just discovered another strange edge case, where my character didn't have enough Magicka to draw power from an Ayleid Well. It says on uesp.net that drawing power from the well is treated as you casting a spell, and therefore exercises your Restoration skill; what it *doesn't* say is that the game checks to make sure you have enough Magicka to cast the spell in question. So, it's possible to get into a situation where you lack the 0 Magicka required to cast the spell, and therefore are unable to use the well to regain Magicka.

The setup to get 0 Magicka is as follows:
1. Use the "showbirthsignmenu" console code to change your birthsign to Atronach. (Optional, but it's easier to demonstrate this if you have Stunted Magicka. Also note that this cheat is well-behaved in Oblivion, unlike in Morrowind.)
2. Fortify your Intelligence; the bigger the bonus, the better.
3. Spend your Magicka casting spells.
4. Allow the fortify effect to wear off; this will cause your Magicka to decrease, possibly in the negatives.

You now have negative Magicka. While in this state, there are a few things that happen:
* You can't cast any spells or use any powers. This includes greater powers; since the cost is 0 Magicka, and you have less than that, you don't have enough.
* You can't draw power from Ayleid Wells, as I mentioned above, because the game treats that like casting a spell, and you don't have enough Magicka for even a 0 mana spell.
* If you are somehow able to benefit from a Restore Magicka effect, or if you don't have Stunted Magicka, your Magicka will recover as normal, but may take a bit of time to recover to 0.
* If a Fortify Magicka effect wears off, your Magicka will increase to 0. (This might also happen if your Magicka is damaged or drained, but I have not tested that.)