The description of the Globe of Invulnerability spell says, in part (PHB, p. 245):
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.
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 on casting a spell at a higher level (PHB, p. 201):
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.
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 spell of 5th level or lower that's cast with a spell slot of 6th level or higher!
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's level the level of the spell slot used to cast it."
Which interpretation is correct? What does the "higher level spell slot" clause of the Globe of Invulnerability spell actually do?