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Diablo III is the third title in the Diablo series (preceded by Diablo in 1996 and Diablo II in 2000). It is developed by Blizzard. A general known issues list is well-maintained and kept updated by Blizzard.
All versions of Diablo III unlock a download of the game on a Battle.net account, which does not require installing an external downloader or client. Retail editions are expected to be cheaper than directly purchasing from Battle.net.
The game is fairly restrictive in requiring constant online access in order to play both single and multiplayer portions of the game, which is a departure from how Diablo II and StarCraft II handled their singleplayer campaigns. An 'always-on' connection requirement for singleplayer can be considered a form of Digital Rights Management (DRM).
Once a digital edition of Diablo III is registered, it is limited to Starter Edition status for 3 days or 72 hours. This has been done to combat economy unbalancing.
The Diablo III Starter Edition provides players the ability to try out a limited demo version of Diablo III. Currently the only way to acquire a Starter Edition is to have a Guest Pass key, of which 2 are allocated per every retail box of Diablo III.  The Starter Edition will only be available with a guest pass until "further notice". 
The Starter Edition has a number of restrictions:
Matchmaking of Starter Edition is only possible with other players of the Starter Edition, and not the full accounts. This includes public matching and private invitations. Counterintuively, this means that Starter Edition players will not be able to play with players with full Diablo III accounts.
The real money auction house has a number of restrictions.
Reportedly working by setting Fullscreen Windowed mode and setting a multi-monitor resolution.
Some settings are held in the file D3Prefs.txt:
Settings can be adjusted, such as:
Using the Darker D3 Pixel Shader by Pyloneer, it is possible to make a change to the brightness and sharpness of the shaders.
Jay Wilson, Game Director of Diablo III, stated the following on 13 May 2012:
Jay Wilson previously stated that altering graphics is against Terms Of Service, and can result in a ban. Following this guide using Darker D3 is at one's own risk.
The FXAA Injector is reported to work with Diablo III to improve graphical sharpness filtering.
You may receive the error message:
Diablo III will block certain systems running the game if it does not detect a certain spec of video card. However, it is possible to force the game to run on any video card (even if it does not meet the minimum specification of the game).
This fix has been acknowledged by Blizzard, but comes with its own warning from Omrakos:
Pressing Esc instead of cancel will make one's computer attempt to load the client.
Pressing cancel will load up a list of unsupported video cards on Blizzard's website.
Open D3Prefs.txt using notepad in the following location:
HardwareClass "0" to HardwareClass "1"
DisableTrilinearFiltering "0" to DisableTrilinearFiltering "1"
Using the program Swiftshader can successfully load the game even on extremely low powered machines. For example, the game can be run on integrated graphics as low powered as the Intel G41, albeit at extremely low framerate.
By default, it appears as if Diablo III has a number of issues with high or custom DPI settings in Windows.
This can result in:
This can be fixed by following these steps:
1) Right-click on the Diablo III shortcut (or Diablo III.exe) and go to the 'Compatibility' tab.
2) Select, 'Disable display scaling on high DPI settings'.
These messages may appear when trying to log in.
This means that no license is attached to the Battle.net account, a problem with many users in Asia. Check this thread for more information.
This message means that the login servers are at full capacity. Official advice is to try logging in later when the servers are less busy.
However, it is possible to bypass Error 37 using this trick:
This might be considered an exploit of the way that the client is reporting Error 37 to indicate that servers are too busy. However, this does not seem to have any particular consequences, except that Blizzard's servers will be busy.
Error 75 means that Blizzard servers are down. Check Diablo III Server Status page for updates.
Blizzard has advised those who have the beta installed, to uninstall the beta and delete all extra beta files, including:
C:\Documents and Settings\All Users\Application Data\Battle.net
Once the files are deleted, run the full retail installer.
Update 8 June 2012: Blizzard appear to have restricted client language switching. Errors 81, 82 or 84 displays: "The client does not match the accounts native language. Please, use the game native client." There is no known fix as of yet, as Blizzard controls the language restrictions from their servers.
Available languages are:
Some players are reporting the game getting stuck at installation in the "Updating game files" part.
The Blizzard Agent may be attempting to connect over the wrong network adaptor. The following steps may resolve your issue (in Windows 7):
Delete Agent.exe in:
Modify Agent.db by:
p2penable : true
p2penable : false
If the user has any connections configured in "Dial-up and Virtual Private Network settings" and they're set to "Dial whenever a network connection is not present," the install won't proceed past 10%.
If the installation has yet to do anything you might need to make sure your secondary logon has been enabled. Do so by going through - Control Panel/Administrative Tools/Services, right click Secondary Logon and proceed to enable it by setting the start up type to automatic.
This is most likely caused by high or custom DPI settings in Windows.
This can be fixed by following these steps:
If one is stuck in fullscreen mode, one can change the D3Prefs.txt settings to force a windowed mode.
Make sure that this line is adjusted:
DisplayModeWindowMode "0" to DisplayModeWindowMode "1"
This error message may appear:
|This Battle.net account does not have a Diablo III License attached to it.
A possible fix is to correctly set the region within the game settings, within the Options > Account > Server Region settings.
Running programs that put an overlay over Diablo III (such as Mumble, Xfire, and MSI Afterburner) can cause Diablo III to crash. Use these overlays at your own risk.
Using settings in the D3Prefs.txt mentioned below one can achieve a reasonable frame rate on e.g. the Macbook Pro Mid 2010 with the Nvidia 330M.
Open the D3Prefs.txt and set the following to options to 1:
D3Prefs.txt on Mac is kept in a hidden folder. To access it:
1) Go to Go Menu at top of screen -> Go to Folder ~/Library/
2) Application Support > Blizzard > Diablo III
Check that you havn't enabled Caps Lock as that disbales ⌘ + Tab, although ⌘ + M still functions correctly.
Game client window becomes unresponsive when a dialog box is opened in the background.
Reportedly working using WINE, with more information on the Diablo III WineHQ entry.
Users connecting through WINE while using Ubuntu 12.04 need to run code echo 0|sudo tee /proc/sys/kernel/yama/ptrace_scope before logging into the game in order to pass the "Authorizing" portion of login. Users who do not run this code will have the game spit out an error message. The code must only be run before the first login since the computer's startup, not every time before logging in, so it's recommended to run when the computer first starts up.
echo 0|sudo tee /proc/sys/kernel/yama/ptrace_scope
A list of general known issues has been published by Blizzard: