Kids like video games. Video games are full of bugs. If you see a bug in a video game, it's often simple to deduce what the bad code was.
Some examples:
- Things which are upside down because someone accidentally typed the wrong z-axis value.
- Pathfinding AI has NPC running around in circles.
- The terrain isn't clipping properly.
- Character models with armor not clipping properly.
- Events or sounds not happening when they should.
- Exploits.
- Animations not ending.
- Placeholder text appearing in game conversations.
- UI / window scaling
- Ammo/item counting incorrectly
- Etc