Ha! Seen hundreds. We used them to boil water to make the tea on the sites.
One site I worked at, theirs never got cleaned out for a few weeks. However when it was, they found a piece of human phoo boiled white that somebody must have thrown in some time before.
Interestingly, I think the lads of that time, instead of skipping the boiler, just threw the phoo out, washed the boiler out, and started boiling the water again. Probably moaned about the taste not being as good?
How do I know? I worked on the site. I still don't know if I drank any of the phoo water in my tea, I can't remember. Probably must have I suppose it seemed to have been in there quite some time.
One thing though, I usually checked the boiler if making tea again.
Imagine the furore with Health and Safety if that happened today?