Did Snape get lucky?

This may be movies-only, but re-watching HBP, the point where Slughorn is offering up Felix as a prize, he says, in the past, "only one student has produced a potion [of sufficient quality to claim the prize]." Since Harry claimed the prize using Snape's notes, I think it's safe to say Snape won the potion the first time. Which leads me to wonder, what was Snape's perfect day?