These are "errors" that happen when a video is compressed too much, resulting in blocky or pixelated images.
In some cases, files that look like MP4s (especially those found in ZIP or RAR archives) can be dangerous. 8327mp4
Interestingly, some people intentionally create these errors for artistic purposes, known as glitch art or datamoshing . Is 8327mp4 Safe? These are "errors" that happen when a video
It uses high compression to keep file sizes small without losing too much quality. The Mystery of 8327: Digital Artifacts and Glitches 8327mp4