The worst rental materials I’ve ever been sent where those the Krenek Double Concerto for Violin and Piano, part the Krenek Piano Concertos project that the English Symphony Orchestra did for Toccata Classics with the stupendous Mikhail Korzhev in 2016-17.
They were shocking, but, in that case, we couldn’t re-engrave them, so my librarian heroically went through everything with the MS, then my assistant and I did the same and caught a mountain more. There were literally THOUSANDS of completely monumental, very, very obvious mistakes. I’ve never seen anything like it. It, they looked like crap.
By coincidence, it was the only one of the Krenek concerti that had been previously recorded, by some orchestra in Germany. Sure enough, when I listened to the recording after finishing ours, every single mistake and omission in the parts was meticulously rendered and faithfully captured. As far as I could tell, neither conductor nor producer and noticed anything amiss (the score was Krenek’s MS, so excuses, I’m afraid), including whole passages missing for very conspicuous instruments.
Only one reviewer actually compared the two recordings. He much preferred the German one, because they had the better English Horn player.
Which is very interesting, because there is no English Horn in the piece….. 
wonderful story! And of course, in the series “Entartete Musik” we had many experiences that were exactly like the one you describe. One experience, I recall as particularly embarrassing was recording an Egon Wellesz Serenade and thinking at the time that even by Wellesz’s standards, the work sounded painfully wrong. Only after the recording was released did a Wellesz scholar contact us and say the publisher had omitted the clef changes.