Undertale

Easily Defeat Muffet:
To defeat this miniboss in Hotland in a single turn just use one of the Spider Bakery items (Spider Donut or Spider Cider) which you can purchase earlier in the game and the fight will end immediately.

Developers Room:
There is a large door in Snowdin that doesn't open for you. However, if you complete the credits without touching a single name you will be able to open this previously locked door and gain access to the developers room.