The worst I've ever actually seen was Eli's breeder.
Most of the rabbits were in 3ft stacked hutches that were in inch deep in filth.
Two of her "favourite" rabbits had free range of a huge ramshackle, drafty barn filled with unwashed milk buckets, rusty metal tools/spikes etc and there was dried wee all over the floor. There was no bed area for them or, from what I could see, access to water other than puddles on the floor.
Eli (a french lop) was in a 2 or 3 ft indoor cage with another frenchie who was bullying him.
she had five or six french lops in one of those 4ftsq playhouses which was in the barn so totally dark inside.
There was a doe with a litter on the concrete floor with no bed or bedding.
There was a rex buck on the bare concrete, no bedding, in a tiny metal pen held down by a gerbil cage with two gerbils in (outside, in this freezing barn).
All the water bottles were thick with algae so I couldn't actually tell if they had water.
There were no food dishes or any sign of them so the food was clearly just being chucked in on top of all the poo. There was no hay.
We only went so my mum could see a giant rabbit and I had the self restraint to leave without taking any of them, I just phoned the rspca and gave them as much info as I could.
Eli had crusty eyes, a runny nose and was clearly terrified of the other rabbit and this woman when I visited so I called my local branch of the rspca and said that they might be getting these buns in and could I adopt him when he came in. They said no so I went and bought him
I later found out the RSPCA couldn't do a thing because there was no home address, just a postcode for a field with a barn.
I know it was the wrong thing to do but I'm so glad I went and saved him.