Overcasting
When you spend more Spell Points than your Casting Limit accounts for, you are Overcasting. This happens when a spell is upcast to cost more than your Casting Limit allows for, or when you use multiple spells or features that use Spell Points in one round and their combined cost exceeds your Casting Limit.
When Overcasting, you have to roll an Overcast Save and take Overcast Damage.
Excess: Your Excess is the number of Spell Points you've exceeded your Casting Limit by.
Overcast Damage: When Overcasting, you take a number of d6 of necrotic damage equal to half your Excess, rounded up. This damage occurs while you cast the spell or use the feature.
If the spell has a casting time of 1 minute or more, this also triggers a Concentration check that you have to succeed on to be able to continue the casting, in accordance with usual spell rules.
Overcast Save: An Overcast Save is a saving throw using your spellcasting ability. The Save DC equals 10 + your Excess. On a successful save, nothing happens. On a failed save, the spell or feature dissipates and you receive one stack of Weavestrain for every 3 you've failed the saving throw by.
When your spell or feature dissipates like this, it has no effect, and the Action, Bonus Action, or Reaction you've used to cast it is wasted. You regain the spent Spell Points.
Examples
A fourth level Sorcerer would like to hurl a Scorching Ray at one particularly tough enemy, but their vision is blocked, so they use Misty Step to reach a better position beforehand. Misty Step costs 6 Spell Points, and so does Scorching Ray, but 8 Spell Points is their Casting Limit, so they are Overcasting with an Excess of 4 Spell Points. They thus take 2d6 necrotic damage and have to roll an Overcast Save, a DC 14 Charisma Saving Throw in this case. They succeed on the save, so nothing happens and they can proceed as planned.
A fifth level Wizard really wants to see the room cleared, and a Fireball for 9 Spell Points would not suffice in their estimation. So they upcast it using 21 Spell Points instead, exceeding their Casting Limit of 9 Spell Points by 12. They thus take 6d6 necrotic damage and have to roll an Overcast Save, a DC 22 Intelligence Saving Throw in this case. They roll a 12, so they fail by 10, taking three stacks of Weavestrain, and their spell dissipates, leaving the room as before. At least they keep their Spell Points, so they can try again next round, with a reduced Casting Limit of 6 due to the Weavestrain.