My GURPS 4e player's book and my Silver Age Sentinels book both fell apart shortly after getting them. For GURPS it was a known issue that many customers had. So many that they sent me a second copy free, along with who knows how many other people. That must have really hurt them financially, and might explain why GURPS 4e is nearly missing from shelves today.
(I'm still not sure if the reason I ended up disliking GURPS 4e was because I didn't like how it almost but not quite measured up to Hero, or how my GM at the time kept making house rules for every character I kept switching to in an attempt to outpace his house ruling...)
For both bad books I bought binding glue and fixed the problems - non-acidic flexible glue from an arts and crafts store.
Sometimes print runs go bad. RPG products are fairly small press, even DnD, so I suspect they're getting second shift treatment at the press. I toured a press once, back when I was taking graphic design classes. The business I went to printed newspapers for one of the local dailies, and slotted everyone else on the side or on spare machines... On reflection, I can see how that might lead to issues.
I cannot say that's the case for DnD's 4E books, its just an idea.
In any print run, no matter how well it goes, there will be bad copies. Somebody is going to end up with those copies. In today's facebook world we can pretty much be sure that the entire population of DnD is online in some capacity, so we're going to hear about it at some point...
Its just a question of scale that will tell us if we're looking at the natural few bad copies or a bad run.