This was early on in my shelving/bookselling career, and I didn't really have the lingo down. I think my response was a meek little: "No, they meant to do that." He gave me a look of incredulity and moved on.
Later, I learned to refer to said untrimmed edge as "deckle." Said with enough authority ("Oh, sure. That's a deckle edge. It's a design choice."), it placates customers.
Powell's blogger, why must you muddy my waters?
Our geeked-out conversation about fore-edges led, of course, down the dark bibliographic road to the term "deckle edges." John Carter defines the term as "the rough, untrimmed edges of a sheet of hand made paper....Much prized by collectors."
Deckle is not merely "untrimmed." It is sort of "untrimmed" on steroids. Here's what deckle edges look like:
(Check the full post for accompanying images.)
I suppose I should go back to saying "untrimmed edge," but "deckle" sounded so much more...official.