If you are trying to launch Diablo 3, and it crashes immediately with “An Unexpected Error Has Occurred”, check the following.

Make SURE that \Diablo III\Data_D3\PC\MPQs and every directory underneath it only contains MPQ files from when you installed it.

I was running the beta, and because my internet is  so bad, I thought I might be able to cheat and use the beta mpq’s in these directories. Turns out the installer does NOT recognise these foreign files, and diablo 3 seems to load everything with a .mpq extension in these directories.

I discovered this these MPQ’s were referenced in my \Diablo III\D3Debug.txt file;

2012.05.18 18:52:07.623826100    OpenMpq – Warning: Data_D3\PC\MPQs\base\d3-update-base-7170.MPQ, not in manifest
2012.05.18 18:52:07.626906800    OpenMpq – opening cache mpq success: Data_D3\PC\MPQs\Cache\base\patch-base-7170.MPQ
2012.05.18 18:52:07.629924100    OpenMpq – opening update mpq success: Data_D3\PC\MPQs\enUS\d3-update-enUS-7170.MPQ

You should remove any MPQ files that read like this Warning: Data_D3\PC\MPQs\base\d3-update-base-xxxx.MPQ, not in manifest

Hopefully this will help you get it working too