AL points us to a story where Andrew Card and Albert Gonzalestried to override the acting AG to authorize their wiretapping scheme:
Comey testified that Ashcroft's wife called him and warned him that Card and Gonzales were coming to the hospital. Comey, suspecting what was happening, immediately headed over to the hospital and called a number of other officials at the DOJ to act as witnesses. Apparently Comey, Goldsmith, and a few others managed to get to the hospital before Card and Gonzales and were waiting in the room when they arrived.
Gonzales had papers in his hand for Ashcroft to sign, but Ashcroft wouldn't have any of it. He said that he agreed with Comey and Goldsmith and that his opinion was irrelevant anyway because Comey was the acting Attorney General. Gonzales and Card then stormed out, and apparently Card placed a very angry call to Comey demanding that he come to the White House at 11 p.m. that night. Comey said he would only show up if he could bring a witness. He showed up with Ted Olson, the Solicitor General. Though Comey is somewhat cryptic about what happened next, he seemed to indicate that a number of high level DOJ officials, including himself, Ashcroft, and FBI Director Robert Mueller, made it clear that they were all prepared to resign if the White House went ahead and authorized a program that the DOJ had determined was illegal.
Comey met with Card alone briefly and later with Gonzales and Olson as well. Though the meeting was "civil" according to Comey, he prepared a letter of resignation afterward. The next day the President called Comey in and talked to him one-on-one. Comey told him to talk to Mueller. Apparently he did and later Comey heard, via Mueller, that the President wanted the DOJ to make whatever changes were necessary to make the program legal.