Ok you are probably on to something I need to apologize for calling him a moron more than once on this.

Come to think about this, we used a lot of different software and switched them as Windows would not open JPEGs by itself I believe.

There's also bit flips. A JPEG can be more easily corrupted if some bits randomly flip.

Also JPEG is lossy, BMP is lossless.

I haven't experienced this with JPEGs but wouldn't be surprised, as I have experienced it with MP3s. Newer decoders can't play some of my MP3s made with older encoders.

Well, a compliant JPEGs will still open in any reader, but can look slightly different. In a low-resolution image, it could change some fine detail you really cared about.

Ages ago I did some bit flip tests on different image encodings for a school project. It was interesting to see how a single flipped bit would propagate across the image in encodings that relied on similarity to nearby pixels for compression. Really the effect was limited to the frame, but the effects could be striking.