At You Don’t Say, John McIntyre digresses a bit to describe his test for copy editing candidates:
Back in the day, when The Sun was still regularly hiring copy editors,* my brutal applicant test contained a short article that had nothing much the matter with it. Any applicant who went to town on it, penciling in multiple corrections or recasting sentences, was automatically disqualified, because such an editor would (a) take forever to get anything done and (b) bring down on my head a storm of complaints from writers about edits that could not be justified.
[…]
*We were repeatedly interviewing and testing because as soon as my hires displayed their mettle, they were promoted off the desk or plucked from the paper altogether by parties like The New York Expletive Times.
This reminds me of the proofreading test I took when I was hired at Monotype (now Six Red Marbles), also in Baltimore. Anxious to do well, I’d been studying the Chicago Manual of Style and boning up on grammar. My prospective boss, Jay Bernarding, slid across the desk a Spot the Difference photo set (which reminded me of the Double Check game on Highlights children’s magazine covers). He told me there were seven differences between the photos and would time how long it took me to find them. I did so pretty quickly (and that’s one reason I got the job), but he later told me that some people couldn’t find all seven, some took more than five minutes, and he was looking for people who could see what was wrong on a typeset page rather than trying to edit text that had already been through two or three passes with several editors and was about as good as it was gonna get.
Click through to see the full-size image. Download it and open in Paint if you suddenly have a burning need to find and circle all 12 differences.