Harry and Fleur have the same lens prescription.
Both in a rush at the Quidditch WC, they bump into each other.
Glasses fall off.
And in the night's darkness they put on each others glasses mistakenly.
Fleur rushes off before they can speak.
Only noticing he has the wrong glasses on when Hermione points it out, he is forced to keep wearing them.
He encounters Fleur in the great hall when she asks for Bouillabaise.
When he turns to see who is speaking.
He sees her, wearing HIS glasses.
They both shout out: THAT'S MINE!