Another haunted level easter egg in 2-4! Wait some time, and a ghost will fly out of the window every few seconds! I'm not sure how this spawns, yet. It's appeared as early as 312 seconds and as late as 400 seconds. Every time it appeared, I beat every enemy, collected every gem, every possible coin, and peeled both posters with the gamepad. I'll keep looking to see how exactly it spawns!