I was thinking of a way to cast haste on an enemy, and then stop concentrating on the spell in the enemy's turn (no action required). The idea is to make the enemy waste 2 turns.
The last paragraph of the haste spell says
When the spell ends, the target can’t move or take actions until after its next turn, as a wave of lethargy sweeps over it. PHB 250
My interpretation of this is that if the caster stops concentrating on the spell, the target will lose his next turn, and if I stop concentrating on the spell during the target's turn, he will lose his current turn as well.
The spell needs a willing target, so we need to "convince" him that he wants that spell. This was covered in this answer Can you make an unwilling creature willing? In other words, what defines “willing”?. We can say that suggestion, charm, dominate and bluff can work to make a target willing.
Some of these spells, like suggestion and dominate person, have concentration. When the caster casts haste while concentrating on one of those spells, he will lose concentration.
This answer (sorry for selfquote), When exactly does a concentration spell end from casting another one?, points out that 2 spells that needs concentration can't be at the same time. When you cast the second, the first ends inmediatly.
The haste scenerio seems a bit different, you change one concentration for another, "you can't concentrate on two spells at once", if this interpretation is correct, the haste spell "switchs" with the other spell.
Also in this answer How does wild shape interact with dominate beast?. We know that when a target is no longer a valid target for the spell, the spell ends. Maybe I'm going crazy bananas here, but that may mean that when the target doesn't want to be hasted anymore (he's not willing anymore), the spell ends. If my interpretation is right, the target will lose only his next turn.
So this opens some questions:
Does the casting of haste fail because the target is not willing anymore? Does the target stop being a willing target and the haste spell will end? Does the spell just work as usual?
And then we have the final question, related to the caster and spell timing, and probably some metagaming on the turn order.
Can the caster stop concentrating on the haste spell during the target's turn to make it waste 2 turns?
Edit: English is not my native language, I'm sorry if things went a bit messy.