Tiling, or corruption, is blocky damage where rectangular regions of a frame are filled with garbage, frozen stale content, or a flat smear, because a piece of the bitstream never arrived intact. It is the signature of delivery loss rather than compression, and it appears on unreliable transport - the UDP/RTP under WebRTC, contribution feeds, and broadcast - where a lost packet is not retransmitted and the decoder cannot always conceal it. Two things make it worse than a single damaged block. First, most frames are stored as differences from a reference, so losing part of a reference frame propagates the error forward until the next keyframe resets the picture - at a 2-second, 30 fps keyframe interval, one corrupted reference can damage up to 60 frames. Second, error concealment has its own look: when its temporal guess is wrong, you see smearing or dragging. Modern WebRTC stacks often freeze on the last good frame until a keyframe arrives rather than show torn content, making tiling and freezing close cousins. A full-reference metric never sees it, because tiling lives in the decoded playback, not the encoded file; catch it with a no-reference check on the decode.
