Term used by domestic builders describing a deliberate error left on a job to distract from a far worse yet less obvious mistakes.
Bob the builder: Sorry about that bit of paint I got on your kitchen counter, Mrs Smith. A bit of turps should get that off.
( Mrs Smith smiles and signs the cheque, failing to notice only three of the four light fittings she has paid for are in place. )
Bob has used the paint spill as a designated distraction.
The person that was initially the designated driver who acts drunk and leaves the bar so the local cop follows him and pulls him over while all the other drunks leave the bar while the cop is busy with the designated distraction.
Bob, the sober one, leaves the bar and pretends that he has a hard time putting the key in the lock...then sits there for a moment leaning over the steering wheel. He drives out of the parking lot of the bar and the cop follows him as he drives just a hair under the speed limit and sways a bit, but still maintains the lane. The cop can't stand it so he pulls Bob over and asks Bob if he has been drinking. Bob says "No officer...I have not." Of course...while cop is busy with Bob...all the other people leave the bar. Then the cop gives Bob the sobriety test and finds out he really is OK and Bob says..."I told you that I had not been drinking...I am tonight's Designated Distraction.