As the volleyball reaches its highest point along its path, it has zero vertical speed but is still accelerating! Photo Courtesy of ArtToday
Yes, it's possible to have zero speed while accelerating, but only for an instant. This situation happens frequently. Acceleration is the rate at which an object's speed and direction are changing with time, so whenever an object passes through zero speed as it reverses directions it has a non-zero acceleration but a speed of zero.
The simplest example is a ball thrown directly upward. After it leaves your hand, the ball rises to a peak height and then begins to descend. Throughout its flight, the only force on the ball is that of gravity, which pulls it steadily downward. The ball accelerates downward even as it rises upward. But eventually the ball's downward acceleration slows the ball to a full stop, after which the ball begin to descend. In that brief instant between ascending and descending, the ball is motionless. It's still accelerating downward during that motionless instant, but its speed is zero.
Answered by Louis A. Bloomfield of the University of Virginia