I live in a block of 4 flats - which share a large garden with block next door.
It's fenced off and our flats have a washing line behind us in enclosed big and one around the side by car park. Literally 2m from garden!
2 2 bed flats and 2 1 bed.
It just happened that we fell into me and downstairs (2beds) using the garden one (our kids were toddlers at the time and safer) and downstairs neighbour (1bed) occasionally uses one around the side.
So we've had a kinda spoken but not set in stone (as such) agreement we use 2 sides each.
New neighbour moved in 2 years ago to other 1 bed and asked if allowed to use washing line which of course I said yes and we generally try through a pretty much unspoken rule to use just 2 sides for all to have equal access.
Noted other 2B neighbour would hang out 10 items (8 on 1 side and 2 on front rung of another side) whenever neighbour used line.
Made me 🙄 but I'm a live and let live type of person.
I work FT. My neighbour PT during school hrs 3 days a week. She knows if I've not hung out washing by the time I leave then line will be empty until next day.
So fast forward to this morning.
I'm hanging out stuff and have loads to do. Was using 2nd side as neighbour cane out house. Said hi etc and asked if she was doing any washing today and if she'd be out for a bit. Yes and yes.
"Great" I said. Told her I was just popping to supermarket but would make most of empty line and Nick extra side - especially as it'll dry quickly anyway.
She got in her car and sat in car park until I'd left. Thought nothing of it.
Came home 1.5 hours later to discover she'd done a wash (10 items) and has hung them 8 on one side and 2 tea towels on outside rung of other side right in middle of strip of line 
She'd clearly hung around and done a quick wash to "bag" her sides. She's gone out now.
She's actually a really nice person and we always chat and say hello etc but AIBU to think she's a slight nut job with CF tendencies who clearly needs to get a life beyond the blinking line 😂