December 30th, 2015
Earlier this year, Wizards of the Coast released the D&D Player’s Handbook errata document to correct and clarify some things in that book. Now they have released errata for the Monster Manual and the Dungeon Master’s Guide.
Some errors slipped into the books, but they have already been fixed in the most recent printings. You have a book that already includes these corrections if its credits page bears the following text toward the bottom of that page: “This printing includes corrections to the first printing.”
Most of the Monster Manual changes are minor math corrections that will rarely have a noticeable impact in play but the changes are included in the document for the sake of thoroughness—to show what’s changed—not because DMs should fret about the minor tweaks. Few of the DMG corrections are likely to be noticed often in play either. Even the meatier corrections—such as the ones in the MM for legendary creatures or the one in the DMG about effects stacking—are within the DM’s purview to apply to a game.