Landing on an error page is frustrating. A little humour or an unexpected interaction can soften that moment and give someone a reason to smile instead of immediately getting annoyed.
This is one place where playful effects make sense: they can help take the frustration out of something going wrong. The page still needs to explain what happened and offer a clear way back, but it can have some personality too.
For my dental website template, I added a grid that reveals parts of a tooth as you move your cursor across it. A small discovery while you find your way back 🦷