Once you create a design you can build it, there is no check to see if it is a valid design. Your not supposed to build anything that is displaying a design error (ie can not be locked).
In fact, it's a good idea to only build locked designs - that way you're protected against accidentally changing them. I thought Steve put in checks to keep you from building unlocked designs, but there might be holes in them. I have a vague recollection that you can't retool to an unlocked design, but if the SY is already tooled for a similar class (so it can build both classes), then there's no check that the non-tooled class is locked. I might be mis-remembering though.
Probably worth a bug report.
John
PS - I've noticed several relatively new users starting threads in mechanics over the last few weeks with questions that are probably more suited to The Academy. Please keep in mind that Mechanics is where Steve tends to post notifications about major enhancements/changes, so it's in our best interest to not dilute his threads (it makes it a lot easier to find that thread you remember seeing 6 months or a year ago....) Thanks!!