I consider it to be basic knowledge that an 11 tooth cog needs a smaller lockring than a 12 tooth cog and if I had made this mistake in my shop, because it is a pretty easy one to do, I'd fix it.
Mind you...nothing gets out my shops door until it has been soundly tested both on the stand and on the road... my customers don't mind a little dust on the tyres as they know I really make sure a bike is 110 % before I call it finished.