Half-Blood Prince

[Carol]

No: HPFGUIDX 183800

Pippin:
>  Snape's association with Sectum Sempra was generally known. Lupin
declares that it's a specialty of his in DH. <snip>

Carol responds:

That statement has always bothered me. Lupin didn't know, any more
than Sirius Black did, that Snape had been a DE, and Severus could not
have performed a Dark Curse that Madam Pomfrey couldn't heal at
Hogwarts without being expelled. (The little cut on James's cheek
doesn't count; clearly he was not "cut always.")

How could Lupin or anyone else know that Snape had invented (or "was
associated with") Sectumsempra under those circumstances? That line
hit me with the weight of a Flint when I read it.

Lupin didn't even know that Severus had invented Levicorpus!

Carol, who knows of no other evidence besides this shaky statement to
indicate a "generally known" association between Snape and this
particular spell