Hostile software
In this case, Apple Preview
I opened a PDF of a Renfe train ticket in Preview. I wanted to remove some colourful advertising from it before printing, to avoid wasting ink, but I was prevented from doing so by document permissions:
It was easy to work around though, right within Preview. I went to print the locked PDF, chose the PDF option in the print dialog, then selected “Open in Preview”:
An apparently perfect copy of the original opened up, but now I could edit it.