Cases of plagiarism in mathematics
The American Maths Society has been caught:

A plagiarised book on maths history.

Another notorious case concerned an Eastern European academician in the bad old days. He began with a bit of self-plagiarism, re-publishing his own works in English language journals. But then he moved on to translating and publishing as his own papers from Russian journals. He was exposed when a Russian mathematician, on moving/escaping to Paris, discovered that his best theorems had been stolen. I think the AMS published an analysis of the whole sad affair.