If a spontaneous caster uses the spell paragon surge, selects Expanded Arcana as the bonus feat, and uses it to learn a spell with a long casting time, what happens if they begin casting a spell with a casting time longer than the remaining duration of the paragon surge effect?
Here is a specific example of the type of situation I'm wondering about:
A 7th-level half-elf sorcerer casts paragon surge and chooses Expanded Arcana as the bonus feat. He selects sepia snake sigil as his new spell known and immediately begins casting it on a rare but nonmagical history text. He is not wearing armor and is in a quiet library on a calm sunny day, and is not interrupted by combat.
However, his paragon surge lasts only 7 minutes. Sepia snake sigil has a ten minute casting time. Paragon surge will expire and they will lose the spell known.
Spell Failure
If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted.
Spells also fail if your concentration is broken and might fail if you're wearing armor while casting a spell with somatic components.
Spell failure states that the characteristics of the spell must be made to conform, but say nothing about whether you must know the spell. That is covered under Choosing Spells:
If you're a bard or sorcerer, you can select any spell you know, provided you are capable of casting spells of that level or higher.
When the half-elf begins casting sepia snake sigil, he possesses the Expanded Arcana (sepia snake sigil) feat, and thus knows the spell at the time he begins casting. The spell's characteristics conform (he is casting the spell on a text of over 25 words in length and is able to touch it), his concentration is not broken, he possesses the material component, he is not counterspelled, and he is not wearing armor. But he would no longer know the spell when he finished casting the spell. He lost it three rounds previously.
So, does the sepia snake sigil spell fail?