Probably an urban myth, but the ‘Chinese Rejection’ letter from publishers is a good laugh:
“We have read your manuscript with boundless delight. If we were to publish your paper, it would be impossible for us to publish any work of lower standard. And as it is unthinkable that in the next thousand years we shall see its equal, we are, to our regret, compelled to return your divine composition, and to beg you a thousand times to overlook our short sight and timidity.”
This was circulated among the editorial assistants when I started working in New York book publishing in 1980. It was printed on a bookmark!