When people expect perfection from themselves, often one little slip-up sends them into a tailspin and back to the old eating habits with a "what's the use" mentality.

It was some sort of trick to get her to slip up.

Or he might not after her accidental slip-up.

Even if he makes an obvious clerical slip-up he is entitled to correct his mistake if he does so within a reasonable time.

The high-flying Swans turned in a solid display to come away with the points, and Jackett was pleased to avoid a slip-up.