C:\Program Files (x86)\StarCraft II\
There are a ton of threads, that boil down to it being a problem with mouse acceleration screwing up when there is a high CPU load.
The problem with turning mouse acceleration off is that StarCraft will turn it back on, even if you disable the option for OS mouse control override.
The solution is a program available here. This program turns off mouse acceleration in StarCraft II after the game boots.
If you receive an error similar to "The file <name of file> could not be written", follow this guide.
<name of file>
1) Right click the installer and choose Run as administrator (skip this step if using Windows XP).
2) Choose your installation options.
3) Once it begins installing, minimize the installer until the installation is completed.
If upon logging into Battle.net you receive the error dialogue "You need an authorized language pack from blizzard entertainment to use this language" use the following instructions to modify variables.txt.
Battle.net Support Article
1) Locate variables.txt within your StarCraft II folder (specifically the one containing your accounts, replays and saves which is distinct from the folder of the same name containing the game installation).
2) Open variables.txt in a text editor and locate the following lines (use control + f to search); localeidassets= and localeiddata=
In each of those lines insert your language code after the equals sign as follows:
! Region !! Code
| '''Americas''' ||
| American English || enUS
| Latin American Spanish || esMX
| Brazilian Portuguese || ptBR
| '''Europe''' ||
| German || deDE
| European English || enGB
| European Spanish || esES
| French || frFR
| Italian || itIT
| Polish || plPL
| Portuguese || ptPT
| Russian || ruRU
| '''Korea''' ||
| Korean || koKR
| '''Taiwan''' ||
| Traditional Chinese || zhTW
| '''Southeast Asia & Australia/New Zealand''' ||
| Southeast Asia English || enSG
3) Save the file and start the game to test.
Restart the game and attempt to login, this will automatically create a new variables.txt file and often fixes the issue although this method may require you to reset any customizations you have made to graphic, audio or control settings (your hotkeys are stored separately and are safe). If you need to reference your previous settings you can use the backup you made of variables.txt.
Deletion of the variables.txt file and re-creation should also resolve any issues with Region Limited licenses that have recently upgraded to Region Unlimited licenses.
This has been found to be as a result of the default Steam shortcut to enter the overlay - Shift + Tab, which makes StarCraft II assume the shift key is being held down. When the overlay is exited, the game does not note the lifting of the shift key, and assumes that the shift key is still being held, thus creating this issue. This can be fixed by changing the Steam shortcut to another key/key combination, such as Insert, Home or Insert+Home.