I think that there is a type of design decision that cause a lot of trouble and trivial benefit.
If designers had been aware of problems, the world would have saved tons of time and money.
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?
The Unix guys originally chose option 2. To them it made sense because it used fewer extra bits and it allowed you to do strings of any arbitrary length. Unfortunately, every single buffer overflow bug is because of that decision.