Backfire In His Face

[EricaWW]

From some conversation among native speakers:

"The plan could backfire in his face."

"Backfire", as used here, means the plan could fail and negatively affect the participant. So what does "in his face" add?

[SoothingDave]

Absolutely nothing. A backfire will be in your face. The expression is redundant here. Either say it will backfire, or say it will blow up in his face.