Where will changes and fixes to guides and manuals for 5e be found? I'm interested in a grappler build, among others, and don't know where to look to see if the last bullet for the grappler feat has been changed. The grappler example aside, it should be useful to know for future fixes.
A set of errata for D&D 5e corebooks have been published, directly to a news article from which they can be downloaded: Errata for the Monster Manual and Dungeon Master's Guide. (Despite the name, the PHB errata can also be found there.) The direct link to the PDFs are here for the PHB, DMG, and MM. This article also contains a link to the Sage Advice Compendium and a few other resources specific to 5e--it's a good one to bookmark!
Along with this, the Basic 5e D&D pdfs have been updated which can be found at the D&D Basic Rules web site.
Several 5e designers have addressed rules questions over twitter—Jeremy Crawford, Mike Mearls, Chris Perkins and Rodney Thompson.1 However, Mearls stated in his reddit AMA that his answers are in his experience only and Crawford is the "official rules expert." Perkins also tweeted that Crawford is the "resident rules sage" and affirmed that Crawford provides the official answers. Crawford confirmed his "official" status in early November 2014, tweeting "Yeah, any official ruling is made or approved by me."
At this time (January 2015), 5e designer Jeremy Crawford is the official source for errata and rules clarifications. Crawford and Perkins have tweeted about an official FAQ (initially slated for Fall 2014 release) and an upcoming 5e rules column on the Wizards website, although neither has materialized. The latest from Crawford is:
Errata documents are definitely on the way, starting with the Player's Handbook. They'll start coming out before the spring. / When I say "before the spring," that means spring is the latest you'll see it. / The errata documents will focus on corrections—adding any missing words and replacing or cutting incorrect ones—not on redesign. (Tweets, 22-24 Jan 2015)
The bullet simply shouldn't be there. It erroneously refers to an old version of the grappling rules.
That part of the feat is an error. It refers to a playtest version of the grappling rules.
- Sage Advice, an unofficial D&D website, collects and catalogs the 5e designers' rules tweets.
The most recent version of the Sage Advice Compendium should contain a list of links to errata for individual books
Frustratingly, Wizards do not appear to be maintaining a single up-to-date reference page which lists all the current errata or how to find it; errata is published in individual articles in the "Sage Advice" section of the WotC website, and those articles do not appear to be updated when later errata affecting the same resources is published. This makes trying to search for relevant errata quite annoying, as the order they are indexed by search engines does not necessarily put the most recent version at the top of the list.
Probably the most reliable way to find current errata for 5e products is to search for the most recent version of the Sage Advice Compendium (as of this edit, that would be the 2020 release, but that will presumably be superseded in the future). The SA Compendium compiles lots of the official rulings and clarifications made in the Sage Advice column and by the lead designer Jeremy Crawford, but most relevantly it also opens with a list of links to individual errata documents for the published books. The relevant section from the 2020 SA Compendium is reproduced below.
Errata have been issued for certain fifth edition books and can be downloaded at the following locations.
Dungeon Master’s Guide
Hoard of the Dragon Queen
Princes of the Apocalypse
Out of the Abyss
Sword Coast Adventurer’s Guide
Volo’s Guide to Monsters
Xanathar's Guide to Everything
Mordenkainen's Tome of Foes
Errata can be found on the WOTC D&D website under the Sage Advice section of the Articles tab. They produce a new article in this category every few months, as needed, and show links to information about other D&D reference stuff there.
Additionally, the Sage Advice website contains answers to rules questions that were asked of Mike Mearls and Jeremy Crawford. Crawford responses were originally considered Rules As Written and Rules As Intended, but this has changed over time and there is no official channel for rulings save the Errata as it is published. That said, Crawford is still one of the more respected voices for RAI within the D&D 5e community as the lead designer of the edition.