Here's another one for the books: When attempting to mail a large Insured Package to a philatelic friend last year, I made the mistake of placing a 1947 CIPEX Souvenir Sheet on the box with other postage to complete the rate.

The Post Office Window Clerk gave the the stamps a puzzled look an immediately informed me that,
"…you can't use those fake stamps to mail this package!" I now began explaining what a Souvenir Sheet was and that it was valid postage; the exchange went on for a while, and then he says to me,
"…even if what you're telling me is true, then why ain't there no price on this stamp!"
OK, now I'm explaining that a Roman "X" means "10" and he still doesn't believe me!
I finally insisted he call the Postmaster to the Window or give the package to a Postal Inspector to settle the matter.
The Clerk finally accepted the package telling me, "
I'm putting this back to have the stamps verified!" I said,
"..fine.." and I left. Four days later the package was received my freind.