In this article analyzing a security failure resulting in live nuclear warheads being flown over the U.S., there's an interesting commentary on people and security rules:

Indeed, the gaff [sic] that allowed six nukes out over three major American cities (Omaha, Neb., Kansas City, Mo., and Little Rock, Ark.) could have been avoided if the Air Force personnel had followed procedure.

"Let's not forget that the existing rules were pretty tight," says Hans Kristensen, director of the Nuclear Information Project for the Federation of American Scientists. "Much of what went wrong occurred because people didn't follow these tight rules. You can have all sorts of rules and regulations, but they still won't do any good if the people don't follow them."

Procedures are a tough balancing act. If they're too lax, there will be security problems. If they're too tight, people will get around them and there will be security problems.