Things like CD's with their large number of partly-compatible extensions shoehorned in remind me that whenever one is writing a specification, one should make sure that every combination of bits/bytes is either valid with defined behaviour, or invalid.

The one exception is a field for "extensions", which should have some bits for 'compatible' extensions (ie. there will be extra data ignored by readers which don't understand them), and other bits for 'incompatible' extensions (ie. you have put a DVD into a CD player).