They, for example, replace digits with others in scans. The replacement digits are layouted perfectly into the page, so the errors are hard to see.

I dealt with this where our fax number had a 6 in it and it would sometimes get changed into an 8, which happened to be a valid fax number for another company, ugh. And this was confidential info too…

Always a funny phone call when they insist they sent it to the number on the cover page we sent and then they send us a copy and xerox made it wrong.