The spell "Globe of Invulnerability" says, in part:
Any spell of 5th level or lower cast from outside the barrier can't affect creatures or objects within it, even if the spell is cast using a higher level spell slot (PHB p.245)
Interpretation 1: For example, Fireball is a level 3 spell, so it doesn't affect anyone inside a Globe of Invulnerability, even if it's cast as a level 6 spell.
Then I read the actual rules of the game:
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level (PHB p.201).
Interpretation 2: Back to the example, Fireball is whatever level slot it's cast with. Fireball in a level 6 slot is a level 6 spell. Level 6 spells affect someone in a Globe of Invulnerability. Therefore, Fireball cast as a level 6 spell affects someone in a Globe of Invulnerability.
Okay, but then why the heck does the spell say, "even if the spell is cast using a higher level spell slot"? There's no such thing as a level-5-or-lower spell that's cast with a level-6-or-higher slot!
I suspect the PHB editors just screwed up with their wording. I suspect that, if I asked a PHB editor what level Fireball was, they'd say "level 3", not "whatever level slot is used to cast it." Heck, that's what I would say. It's only because of spells like Counterspell, Dispel Magic, and Globe of Invulnerability that I'd pause and say, "Actually, no, 3 is the MINIMUM level slot that can be used to cast Fireball. Per the rule on p. 201, a spell is the level of the slot used to cast it."
Any other insights on this?