Funny, I used to work with a copywriter who would always leave a simple punctuation mistake in her work so her editor (me) would have something to do. I think she knew if I had to struggle to find something I'd probably suggest unnecessary rewrites.
In graphical design I've heard this being mentioned as the "Pink Elephant", i.e. "obvious misses" that the client can easily clank down on. This makes it easier to keep the integrity of the overall design.
In publishing, non-obvious mistakes are used to detect copying.