To my understanding, misprints happen quite often but do not ordinarily increase the value of an item... in fact, they tend to do the opposite. Exceptions exist, naturally, but this tends to be because the item was recalled/destroyed or very very few exist, and they're interesting to collectors. I'm not sure yours qualifies in that department but I am also sure someone somewhere collects misprints.

I've never seen it, but I've also only ever had one copy of the box set. Just never floated my need. You could probably get around $5-10 for it... not sure what the set usually sells for, but I would not be (how I judge value) interested in it for much.

What he  said.  Printing errors such as this happen on more items that you might suspect.  Probably on anything that is for 2nd Edition D&D or later, these misprints would decrease the value of the item.

I'm sure someone must collect misprints as well but I have no idea who it is.  I will check my copies when I get back to my collection and see if occurred in the whole print run or not.  I think Ruins of Undermountain has at least 3 different prints.