Whenever Pepe tells you that you've opened a new level or boss.
The chime that plays when you collect a Muppet Token.
That One Achievement: Racing Willie in Escape Claws. He's wicked fast and it doesn't help that the level has a lot of bottomless pits, leaving you little room for error.
That One Boss: The Goulfriend's boss fight where you slide down a hill three times and try to avoid the holes she punches in the earth. Not surprisingly, each part is more difficult with the third involving penguins pushing a block of ice your way.