It happens frequently. Some years ago a bullet came though the window of my mom's third floor apartment. From the hole in the window and the impact of the bullet in the floor, it had arrived at around a 45 degree downwards trajectory.
More here, also a fun site.
http://www.frfrogspad.com/miscella.htm#straight
Hatcher’s tests indicated that on the average, vertically fired rifle bullets reach about 9000 feet in altitude (slowed from their muzzle velocity by air drag and gravity to zero velocity), taking about 20 seconds to reach maximum height. Then, pulled by gravity, and slowed by air drag they take about 40 or so seconds to return. Bullets fired vertically come back base first. Why? Read on!...
The Haag article used a ballistics computation program to calculate vertically fired bullet performance and came up with results comparable with Hatcher’s work. Using bullets ranging from the .22 rim fire to the 180gr .30 caliber spitzer in the .30-06 the time of flight (up & back) ranged from a low of 25 seconds for the .25ACP to a long of 77 seconds for the M193 ball. Maximum altitudes ranged from a low of 2288 feet for the .25ACP to a high of 10,103 feet for the 180gr .30-06. Terminal velocities ranged from 134 f/s for a tumbling .22 Short to a high of 323 f/s for the 180gr .30-06...
As a point of interest, a velocity of about between 180 and 360 f/s (±) is needed to penetrate skin. The wide range comes from the non-uniform strength of normal skin tissue. Projectile shape has no statistically significant effect on the penetration. However, one could still be seriously injured if struck by a falling bullet even if it doesn’t break the skin.