zalt,
It's called "requirement creep", and it's a problem that causes schedule slip and cost overruns on every defense program. Both sides (customer and supplier) usually share responsibility. The customer always wants more performance, bells and whistles. And the supplier is usually happy to oblige since they will get paid handsomely for the changes.
Here in the US, DOD programs are now subject to the Nunn-McCurdy amendment, which calls for termination of programs that exceed the original budget by 25% or more (unless, of course, that particular program is deemed "of significant national interest").