As far as Ned’s mistakes go, what about his general truancy? He had a seat on the small council for 14 years, as Warden of the North, that he never claimed. He helped make Robert a king, but showed no interest in following up and keeping an eye on how the kingdoms were being run–did he even keep correspondence with any of the River or Vale lords?–and so he’s completely floored by the state of Robert’s government as soon as he can’t hide from it anymore.