Or I get hit during combat? Or something else funky happens?
How often does this question come up?
I have a very simple, abstracted way to handle this.
The character in question must make a save vs. spell, with a penalty to the save equal to the spell’s level. The only modifiers allowed to the roll are things like Luckstones, Talismans of Protection, spells that have similar effects, etc. Magic armor does not help you here… There might also be some extra penalties depending upon circumstance, but not very often. If you are missing both material and somatic components, double the penalty for the spell level.
As spell-casters go up in level they get better at it, and it is more and more difficult to do with higher level spells.
So, hit in combat? Roll to see if you lose the spell. The spell is disrupted no matter what, but you might not lose it.
Hands tied and trying to cast a spell with a somatic component? Roll for it.
A priest without a holy symbol? Roll to see if faith alone gets you through.
Trying to cast while drunk? Roll to see, with an additional penalty equal to Attack penalty for the level of drunkenness.
The one hard-and-fast rule I have is that both Arcane and Divine magic absolutely requires a verbal component, if you are gagged you are screwed. I also generally don’t let this happen for the sorts of big ritual spells that likeSpiritwrack, etc.
Nice and simple.