Multi-Platform Battlefield

You: "My app works on all platforms!"

Reality: Someone's trying to run your code on their Samsung smart fridge and suddenly your medieval knight armor doesn't feel so impenetrable anymore.

The eternal struggle of "write once, debug everywhere" continues. Your app might support Windows, Mac, and Linux, but there's always that one user with a toaster running Android 2.3 wondering why your UI looks like abstract art.