"Behind him, the Impediment Jinx was wearing off. Snape was beginning to inch towards his fallen wand, spitting out soapsuds as he crawled. "I wouldn't go out with you if it was a choice between you and the giant squid," said Lily. "Bad luck Prongs," said Sirius briskly. "OY!" But it was too late; Snape had directed his wand straight towards James; there was a flash of light and a gash appeared on the side of James's face, spattering his robes with blood. James whirled about; a second flash 1/?
-of light later, Snape was hanging upside down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of graying underpants.“ OotP, Snape’s Worst Memory, pg 647. Considering the circumstances, I do not see the use of the spell unjustified.
But we also… don’t know that it’s Sectumsempra??? Snape doesn’t use an incantation, it could just as easily be another spell. But I agree; if that’s the moment that the OP of that post was talking about, then I hardly find that unjustified - James and co. went after Snape, who was doing nothing to them, and attacked him?? Used hexes on him?? Snape’s defending himself??
Although I still call bullshit on that Levicorpus thing. And I’m not entirely convinced that he’s using Sectumsempra here.