[Suicide Junkie]

Are you are trying to retrofit to a design you have JUST created on that same turn?

If so, when the game gets processed, the design ID of the order is probably pointing to the wrong design.

For example (this is likely not how Aaron implemented it, but it shows the problems that the game has to face)

Player #1 has 5 designs, A,B,C,D,and E.
Player #2 (you) has 5 Designs, z,y,x,w, and v.

The host sees these designs as #1, through #5, and #6 through #10.
If both players create a design on the same turn, they will think their own design is #11 until they are told about the other player's design.

So, if you give a ship an order to retrofit to your new design, it will be an order to "retro to design #11"

When the host processes the turn, it will have to pick one of those designs to be #11. Say it picks Player #1's. Your design is given #12.

Now, your order is followed. "Retrofit to #11".
Whoops. That's player #1's design. And it happens to be a fighter or a base, or a different size ship.
Thus, the order fails.

If Player #1's design happened to be of the same size, and within a 50% price difference, you could quite possibly succeed in retrofitting to a wierd design that you can't build.
You will then end up with a useless ship containing big gaps that you do not have the tech levels to repair.