For years, I've believed that recompressing JPEG files multiple times would gradually degrade its quality until they are an unrecognizable mess, the way making photocopies of photocopies does. This intuitively makes sense because JPEG is a lossy format. There are also other Q&As that claim this is so:
However, I've also read that recompressing JPEGs at the same quality level will not degrade the image quality. This runs counter to the gradual degradation that is described elsewhere.
What technically happens when a JPEG is recompressed? What is being lost and how? Will the image really transform into the snowy mess that used to appear on television? What about those videos showing images that fall apart after being recompressed multiple times?
(Please don't merely handwave and appeal to the general concept of lossiness.)
(This question, and the answers it has attracted so far, focus on the technical factors (specific settings and image manipulations) that cause or prevent image degradation when a JPEG file is recompressed multiple times.)
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