Flying creatures need a minimum forward speed to be able to fly.
Minimum Forward Speed: If a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement.
So let's take a harpy with a base fly speed of 80ft (average). and imagine that she flies towards a wizard who has readied Gust of Wind to be triggered just before the harpy reaches the him. The harpy has already moved 60ft and is then blown back 1d6x5ft. by the Gust of Wind effect.
Gust of Wind: This spell creates a severe blast of air (approximately 50 mph) that originates from you, affecting all creatures in its path. Medium creatures are unable to move forward against the force of the wind, or if flying are blown back 1d6×5 feet.
Now what would to the harpy?
Would she stall immediately because her forward speed dropped to zero by being blown back?
Would she continue with her remaining 20ft. of movement and then stall (or be forced to use her second action as a move action)?
Would she continue with her remaining 20ft. of movement, not stall and be able to take a standard action?
I suppose RAW the latter would be the case, because she has done 40ft. of movement necessary to keep flying in her round.
What about other spells / powers? Imagine the same situation with the following:
Telekinesis to grapple the harpy, winning the check and making her unable to move.
Psionic Blast which stuns the harpy, making her unable to take an action.