Most recently would be the Maw.
The game's colorful and rounded, the gameplay is easy and very forgiving, the story is simple. It's has all the earmarks of a nice sweet kid-friendly game, but late into it I realized the Maw consistently got bigger with each stage and was starting to get freaked out by the fact I'm nurturing a monster into maturity. I kept wondering if the people who made the game designed it like that intentionally. The ending just kind of freaked me out when it seems like that's may of what the developers were going for.