Such as a komura super wide 47mm. I think there must have only been 100-200 of those made.
I think currently my Olympus Wide-S with 35mm f2 is the rarest I own. They only come up a few times a year. I'm not sure how many were made. I estimate less than 10000 but I'm not sure how less.
I find it interesting that really rare items don't necessarily fetch high prices. Maybe if there aren't many around, not many people talk about them.