In my early games I used to design colony ships without engines.
I once designed a dreadnought, a ship that would save my empire, and in order fund this endevor I would scrap some of my ships each turn while my uber-noughts were being built. Imagine my surprise when 50 of my uber-noughts went on line and I scapped the rest of my fleet just to find out that all of my new ships had no engines and would not move.
Retrofit time.