Back burners not fully operating.
Bad spot on the disk.
Bats in the belfry.
Batteries not included.
Been napping in front of the ion shield again.
Been playing with the pharmacy section again.
Been short on oxygen one time too many.
Been using her head as a mass driver.
Blew the hatch before the lock sealed.
Blocked one too many hockey pucks / soccer balls / punches with his head.
Blown/leaking head gasket.
Born a day late and like that ever since.
Born during low tide in the gene pool / swimming in the shallow end of the gene pool.
Both oars in the water, but on the same side of the boat.
Brain transplant donor.
Bright as a Zippo lighter without a flint.
Bright as Alaska in December.
Bright as an acetylene torch -- without an oxygen supply.
Bubbles in her think tank.