After a battle where I was having my ranger cast spells, she was basically spent in terms of spell points. Note that I used Superman (via the Invigorator Belt), which gave her some temporary extra SP.

After the fight, she had 0 SP (actually negative, but the game displayed 0) for Fire, Water, Air, and Earth, and only 5 SP for Divine. When I rested, it took a moment for her magic bar to really recover because her spell points had to get out of the negatives, and the user interface treats such values as zero.

The effects of Piety on max SP seem a little odd. In testing, I seemed to notice the following results (having my Bishop (who has Power Cast) use level 5 Superman on my Ranger):

Fire: 38 to 49 (+11). She has 9 skill and knows 4 spells.
Water: 43 to 55 (+12). She has 8 skill and knows around 7 spells.
Divine: 40 to 49 (+9). She has 17 skill, but only knows 1 spell.

Why did she gain fewer divine spell points? This is something I will need to look into.

By the way, I have also had negative spell points in Morrowind, but in that game, you can get rid of them by saving and reloading. (That game is full of exploits, many of which are much bigger than this one.)

I was wondering if negative spell points can cause a crash. I've had a few occasions where I was really out of spell points and casting spells. I run into some savant drone that were draining spell points.

It looks like the game will allow you to cast the spell, even if you just got drained on a spell area you are casting from and the spell would put you negative.
Is this what actually happens?
1) CRASH!!!
2) Cast spell anyways but end up in negative spell points.
3) Spell cannot be cast as you no longer have enough spell points. FIZZLE
I'm not entirely sure though.

Here is an example.
I had 9 spell points left in fire domain. I set to cast haste that will cost 8. I get hit by a neural blast and lose 6 fire spell points.

I believe what happens is actually:
4) Spell is still cast, but at a lower power level. If that is not possible, the character defends instead..

However, I actually have had a crash from negative spell points. For this to happen, you need to cast Remove Curse from the inventory screen, have the spell backfire, have the character fail to resist the backfire (and be hexed as a result), and be sent into negative spell points as a result; the game will then crash with an assertion error. I posted about this in another topic.

In case anyone is wondering, negative stamina is possible: My Gadgeteer got hexed when low on stamina, and her status screen showed her stamina as -32.

The Gadgeteer was still conscious at that point.