I'm not certain whether this is an error of the writer (possibly distracted while writing?) or of the typesetter/designer, but either way, a good copy edit should have surfaced the error before it was published.
When you're editing, you need to check your text in a few different ways:
1) Is the content factual?
2) Are all the words spelled correctly, numbers presented accurately, etc.?
3) Is the grammar correct?
Errors can creep in at any stage of the process
I was shocked the first time I received a "final proof" of an annual report from a designer only to discover new errors had surfaced in the text. They were minor changes - in fact, they were instances in which the designer had decided to "help" my client by "improving" the sentences (note the use of sarcastic quotation marks - see last week's post on this blog).
We caught them all and changed them back before it went to print - but only because I was proofreading the whole text one more time (as opposed to simply confirming our last round of changes had been made accurately).
Once the document is published, it's published. You can sometimes withdraw it to correct an error - but often, you can't. My advice: always give a text one full, final proofread before signing off.