Sometimes the workshops were amateur and careless. At other times, material was more carefully graded. For example, clean covers with good cancellations on the stamps might be taken out and only covers with poor cancellations cut up and the stamps washed off the paper. I have been told that in some cases, workers were paid a bonus if they found errors or varieties - probably they were told what to look for.
Yesterday, I was looking through old Bundleware and "Used Off Paper" from the Philipp Schmidt collection - recently sold as the "Krim Sammlung" by the Bamberg auction house Arbeiter. This has been locked away for decades and was collected beginning in the 1930s - when an old glassine gives a dealer's address as "Adolf - Hitler - Str" it helps date the collection ! (Click on Image to Magnify)