Ok, this isn't actuall an SEIV moment, but an SEIII moment:
I was in the middle of a war, my ships getting pounded and my little three system empire getting hammered. I hadn't built any new ships in a long time and my carriers were about worthless, so I design a new ship class, way better tech than my current fleet. Set all my shipyards to build it. They get constructed, I set the entire fleet to move to my nearest warp point. Nothing happens. I look at the design and realise that I was short 1 engine.
I was so mad!
(on a side note, did the same thing SEIV, this time forgot to give it any engines, only built three ships fortunately.