A patch is never enough...

[Suicide Junkie]

I created a ship in an QNP mod (mine, actually, that I was testing)that was almost entirely filled with engines... it showed up as having 20 or so movement in the design window, but when I ran it in a combat simulation it only had 3! Is this some kind of overflow error?

This may be caused by the following:
Your ship was designed with a standard movement / engines per move = 20. The design therefore showed as 20 MP.

However, if there was no lifesupport components required or added, then the ship would suffer a 1/4 movement rate in the real world.

20/4 = 5 and in combat, 5/2 rounds up to 3 mp.

THe same thing would happen if you had lifesupport but no bridge or crew quarters.