For example:
Big-Endian and Little-Endian:
The difference of endian forces developers to be careful of target architecture. If early processor vendors had communicated and established which is the standard, there would have been less trouble and no need to invent byte order mark(BOM).
32bit IP address:
Of course it is difficult to judge how many devices will be in the world, but sometimes I have an uneasy feeling about shifting to IPv6. I think that the industry will experience confusion and need to devote much effort than we think.
What do you think is most regrettable design decision in computer industry?
