A duel in the Trophy Room turns into an encounter with Fluffy

"If either of you get us caught, I'll never rest until I've learned that Curse of the Bogies Quirrell told us about, and used it on you."
-- Ron Weasley to Hermione and Neville (PS9)

Draco challenges Harry to a duel the evening of their first flying lesson. Draco suggests midnight in the Trophy Room. In reality, Draco is setting Harry up to be caught out of bed after hours and has no intention of dueling anyone. Harry, with Ron in tow as his second, heads out of the Common Room only to be followed by Hermione who is telling them off. They encounter Neville, who couldn’t remember the password to get back into Gryffindor Tower. When Hermione can’t berate Harry and Ron into giving up the dueling idea, she finds that the Fat Lady has left her portrait. So all four of the First Years venture out into the castle in search of the Trophy Room.

Draco isn’t there, of course, but Filch has been tipped off and is poking around trying to catch Harry out of bed. The Gryffindors run for it, are spotted by Peeves, and end up in the forbidden Third Floor Corridor where they open a door and encounter a three-headed dog. They flee in panic and make it back to Gryffindor Tower. Hermione, although furious with the others, does note that the dog was standing on a trapdoor, which turns out to be a clue to what’s really going on in the castle.