One Disc Lied — How do computers find a mistake in a message?

Computers find mistakes in data by sending a few extra check marks along with the message. If one bit gets flipped on the way, one row check and one column check break - and the mistake is sitting exactly where those two cross. Read a wooden signal board across a harbour at night, walk to the wrong post because one disc turned in the dark, and then learn the trick that lets you point straight at a lie you never saw happen. This is how a QR code still scans with a corner torn off.