A small bomb exploded early today on the sidewalk outside the offices of Northwest Orient Airlines in downtown Miami, apparently placed by mistake.

No one was injured in the 12:30 a.m. blast.

The office at 300 SE 1^st Street had windows shattered and an estimated $500 damage. Northwest has no relations with Communist Cuba and authorities believe the blast was really meant for another airline office in the same neighborhood. It's believed that Cuban exile bombers, suspected of scores of blasts in recent years, tossed a time bomb at the wrong building.

Miami police said the device, with a short acid pencil-type fuse was dropped behind a hedge at the corner of the office.

Police said late this morning that no exile group had called to claim credit for the explosion as is their usual habit. A similar incident, at the Air Canada ticket office at Collins Avenue and 70^th Street Friday, did an estimated $20,000 damage to the recently renovated office and seriously damaged five neighboring shops and two parked cars.

Earlier this month, the FBI arrested nine members of the secret exile terrorist group called "Cuban Power," which was believed responsible for many of the bombings that have rocked the homes of foreign consuls and businesses that have indirect dealings with Communist nations.

The nine Cuban power members arrested by the FBI, however, were charged with the shelling of a Polish freighter that was docked here in early September.