I don’t think I’ve come across that before, but I’d say it depends on what is meant:

• I don’t know what that thing is.
• There is a thing, but I don’t know what it is.
• There is a thing such that I don’t know what it is. I.e., I do not know what all things are.

There may well be some other ones, but I don’t know what they might be.

I have a Xerox colour laser printer that I’m very happy with: accepts off-brand toner, speaks postscript, good quality printing, no problems at all. I’ve also been very happy with Brother laser printers in the past.

Oddities and Curiosities of Words and Literature by C C Bombaugh, one of my favourite reads, feels like it might be an obscure book.