Quite a lot of hardware devices have broken encoders. I noticed just how inconsistent it is if software will tolerate invalid files or work around it.

Seems these days a there’s more of a preference to outright refuse invalid files since they could be exploit attempts.

The interesting thing about the situation I mentioned is that while the encoding algorithm was wrong, the actual output was valid JPEG that simply didn't look quite visually correct. But you're right, invalid encoding can be a problem too, and I have noticed during testing that a lot of decoders are quite forgiving of it.