Upstairs flat- they were the freeholders of two other flats in the building - we wanted to take out a wall (with structural engineer plan, fully insured etc) among some other works. Well, husband was determined to take out the wall, I didn't want to, but that's a whole other thread. Told upstairs neighbours what we wanted to do, they said 'OK, fine, we'll just need to cross some i's and dot some t's' and we said we'd share all emails, plans etc.
Got started and the demands started coming - that the builders agree to take on a liability for the structural engineer's plans, that we do this, do that, produce a full-grown unicorn for their inspection. Freeholders are not supposed to 'unreasonably withold' permission for works and we were clear everything we were doing was insured and done by qualified people.
They eventually send us a 'cease and desist' notice for the building works when we were without a kitchen. DH rebuffs it with the fact the lease states the dwelling cannot be left uninhabitable, which is is without a kitchen and we carry on, but in the end we drop the knock through (I'm kind of relieved about that)
Oh, did I mention I was also nearly 7 months pregnant at the time?
After this, they handed us a large bill for their lawyer's fees. I paid it some time later before we sold so we could settle the dispute before going to market.
The funny thing was we sold the flat eventually to a guy who was a property developer buying it for a relative. He wanted to, and indeed did, carry out a far bigger building project on it than we planned, which upstairs must have hated, but presumably he had better lawyers and was bullish enough to stand up to them. That was quite a nice bit of karma (NB, we didn't know he'd do that when we sold to him)