My attempt to explain how STARKs catch an error: Imagine a kaleidoscope. The beads inside it represent transactions: white for valid, red for invalid. Let's say this kaleidoscope had a thousand white beads and just one red bead. That single red bead would be reflected in the mirrors again and again. Its image would bounce across mirrors, then into mirrors reflecting those mirrors and so on. The result: it would appear everywhere. You wouldn't be able to miss it. This is somewhat like what STARK proofs do: a single tiny error or invalid transaction becomes visible across the entire proof until you can't miss it.