I've seen enough modules pass through my hands to know that there must be a reason that every once in awhile I get one with a tiny "cut" on the spine (usually a few inches down from the top).

The cuts are always the same and affect the interior booklet as well.  What is the signficance of them?  Why were they put there?  

Many of the modules that came in the 10th Anniversary Bonus packs were notched.

It's an "overstock notch".  I'm not exactly sure why it's done (why it's important to delineate overstock from normal stock), nor do I know whether it was done by dealers or by TSR (or both).   It's a fairly common practice in the bookselling field.

Foul

They are also in the form of holepunches in the upper corner. A lot of the old Wee Warriors/Little Soldier-style items were hole punched.

FoulFoot wrote:It's an "overstock notch". I'm not exactly sure why it's done (why it's important to delineate overstock from normal stock), nor do I know whether it was done by dealers or by TSR (or both).  It's a fairly common practice in the bookselling field.

Foul

Because the overstock stuff is often sold to dollar stores and the like for whatever price the distributor can get for it if it doesn't sell out the first time around.  As an example, I had bought a whole bunch of DC Heroes stuff for a dollar a book many many years ago.  I don't have any of it anymore, having gotten rid of my DC Heroes stuff, but paying 1/10th the cover price for support material for a game I had was a good deal.

CD's and records are notched like this as well.  Paperback books are a bit of a pain.  Sometimes the front cover is stripped off the book and then the book is sold.  Usually that's not too kosher as that book has been reported as unsold and destroyed, the purpose behind taking the front cover off the book in the first place.  Hardcovers generally do not get stripped, but if they do, then the dustjacket, if available, is taken.