6.2 Merge window, part 1

“ The eight S-boxes of DES were the subject of intense study for many years out of a concern that a backdoor (a vulnerability known only to its designers) might have been planted in the cipher. The S-box design criteria were eventually published (in Coppersmith 1994) after the public rediscovery of differential cryptanalysis, showing that they had been carefully tuned to increase resistance against this specific attack. Biham and Shamir found that even small modifications to an S-box could significantly weaken DES.[4]”
From Wikipedia. From your description, you know more of the details of S-box creation than I do, but this sure sounds like an S-box could be weak in a way that might not be apparent to others.