Neville assumed he'd come upon two fourth years in a scuffle over who'd stolen whose last chocolate frog, and it was with every bit of his newly mustered confidence over being named Prefect in Ron's absence that he rounded the corner on Pansy and Ginny, the latter red faced and in a rather compromising position, and Pansy clearly in the middle of some abuse of power.
His face hardened unexpectedly, so angry for a moment he couldn't even speak. He wanted to hex Pansy into the opposite wall, but knew that wouldn't do any good.
"P-p-professor Carrow wanted to see you, Ginny," he said hurriedly, the lie bold and poorly managed, but likely no less socially inept than Pansy expected from him, which was in Neville's favor. "She told me to come and get you."