Very small fighters can potentially have an Armor Rating of X-0, where X is the layers, but 0 is the columns. This throws up a divide by zero error when they are selected in the Fleet Organization window.
Likewise, engines that aren't strong enough can provide a 0 km/s speed, this also throws up a divide by zero error when they are selected in the Fleet Organization window.
In both cases, such ships have the word "error" in the Ship / Class Design screen on the Fleet Organization window. @Conventional Advanced Composite, the armor bug occurs at >30 tons. For the 0 km/s bug, just slap a 5 Ton engine on something and start adding components, you'll get there sooner than later.