Awesome, but Impractical: Given just how many tinkerers there are (see: Home Inventions) around the place, this happens a lot.  Fortunately, the Imperial Military Resource Board and its commercial parallels are pretty good at filtering out the ones impractical enough to get people killed and battles lost before they hit production.

For modern examples – well, rather than repeat myself, go check out More Dakka and read the last couple of examples: the Dyson laser that can’t be aimed in any sort of reasonable timespan and the antimatter-grenade Gatling-gun.  There is a reason why you will never see either of those on any canonical battlefield; this trope is it.