As a practicing structural engineer for 30 years who also writes structural engineering design software, let me answer this for you. When an engineer signs/stamps a design, they are not certifying that it is perfect. In fact, it is generally recognized that no set of plans is ever error free. What you are asserting by your signature is that 1) You were in charge of and supervised all the work going into the design and 2) The design was performed in accordance with the standard of care for the work being performed. Standard of care is NOT a standard of perfection, by rather "that degree of care and skill ordinarily exercised under similar conditions by reputable members of our profession practicing in the same or similar locality." TL/DR; An engineer cannot sign/stamp shoddy work, but is not expected to perform perfection.