We were renting when we found our current (perfect!) house - in mid-contract, so I read through the lease and was v pleased to find a clause that stated we could end the lease with two months' notice. Once we'd exchanged on the new house, I dropped off the required notice in writing to the agent and continued planning the move.
A day or so later, I got a call from the agent, somewhat confused, to say we still had five months left on our lease and did we know that? I pointed out the clause in the lease, which he read with disbelief and then told us his employee had made a mistake (!) and accidentally included the clause when she should't have. I said, "Oh dear, that makes things very difficult for you, doesn't it?" and left the ball in his court. We'd both signed it - it was his problem.
Except it became our problem. The property owner had 2 properties, one of which he'd been living in, and had planned to move back into ours once our lease expired. He was furious at having to move early, because he didn't yet have a tenant for the other place and didn't want to lose any income. He and the agent had a huge stoush, and agent ended up finding someone to take the other flat in a hurry... such a hurry that they were going to move in 2 days before we were to move out.
After this, things really went downhill. Landlord asked if he could move a bunch of pot plants into the garden a few weeks before - I said okay. He then asked if he could park his multiple cars 
on our drive the week before - I said okay.
The day of the move was a total fuckup from beginning to end. The movers we'd hired (with packing service) took SIX HOURS just to get our stuff into boxes, wrapping each piece carefully (and sloooowly). Then they'd brought a smaller van, so it was going to take more than one trip. It was five days before Christmas, so dark at 4pm - and in the middle of it all, agent rings me to say landlord wants to start moving his stuff in that night (!!) so can I let him know when he can come do the moving-out inspection ASAP?
I said things were going v slowly, and we hadn't had a chance to clean anything yet (movers had tracked in an unbelievable amount of dirt). Technically, we still had two days left on our lease, so they were really pushing it. Agent reassured me (via text, so in writing) that he would do the walkthrough with me, and we wouldn't be liable for anything after that point. So I thought, "Hell, if he really wants to move into a filthy house, so be it," did the walkthrough with the agent (all clear), left money for the cleaners as a gesture of goodwill, and proceeded to the other half of my hellish move.
Two weeks later, no deposit had come back. Contacted the agent, and he said owner had "a few items he wanted compensation for." Turns out the owner had spent those two weeks going over the house with a fine-toothed comb (even pulling up carpets to check the underlays!) and had a huge laundry list of over £2000 in "damages" he said we had caused. All this despite the fact that the agent had inspected and given us the all-clear (something I'm pretty certain he never told the owner he had done).
In the end, we didn't proceed to arbitration, though it was a close thing and I still think we should have. He got around £500 from us he shouldn't have had, and I'm still hopping mad whenever I think about it. Effing twat - I raise a middle finger in the direction of the house whenever I drive by. 