In one of their attempts to scare Courtney, it involves luring her to the treehouse in the woods. To do so, Molly would call her and pretend to be Denise and ask to come with her to the treehouse. And even though she sounds exactly like Denise, the plan backfires terribly because Courtney tells the caller over the phone that the real Denise is standing right beside her.
When Courtney sees that the Mud Monster is indeed standing behind her, she takes a moment to calmly admit Eddie and Hat were right before she talks it into submission.