Sometimes even the directors of a quality show like Northern Exposure fail to film an important transition shot or angle, and, says coproducer Martin Bruestle, ''by the time we realize that we are missing it, we're already filming one or two episodes down the line.'' In those cases, says Bruestle, they resort to what he cheerfully calls ''the magic of editing.'' We call it covering up your goofs and hoping nobody notices. ''Sometimes we get busted,'' admits Bruestle. Herewith, a couple of notable gaffes we caught this season.
· At the Brick, a blind piano tuner (Kevin Conway) has taken apart the resident instrument. An instant, as bar owner Holling (John Cullum) looks back at the piano, it has been rebuilt, and the tuner is wearing different clothing.
· Working at the Brick, Shelly (Cynthia Geary) is in a skirt and a white top, her hair up. But in one shot, her hair is suddenly down, her top is green, and she's in pants. Says Bruestle: ''Would you believe Shelly is an incredibly quick dresser?''