The way their warehouse worked was that the ERP system would generate a document for each sales order as they were processed, and then send it to that warehouse line printer. After a warehouse worker filled an order, he signed the document, put it in a basket, then went to the printer to get his next one. Most of the time there would be another one already printed and sitting there, so there was no problem. But on occasion James would get there while a document was still printing, so he would "speed it up" by pushing the Line Feed button a few dozen times in quick succession -- no doubt trying to look busy because his boss, the warehouse foreman, was right there.
That's the reason we had such a difficult time figuring out the problem. It was very sporadic and only happened when James was the one (of about a dozen other warehouse workers) that got there if by chance it was in midprint. And since James would often walk away after pushing the button, another warehouse worker would sometimes come in and pull the document off the printer, see that it was illegible, and show the warehouse foreman, who then had to reprint the document.
Anyway, mystery solved.
I've been in the IT consulting business for almost 20 years, and have seen dozens of situations like this -- it seems like a lot of employees think only about the job at hand and go into drone mode techwise when they get to work. Our high-tech solution to this particular problem? We put a sign above the printer saying, "DO NOT TOUCH PRINTER WHILE IT IS PRINTING!!!"