Perforce SCM Failed To Load in Visual Studio 2008

This was odd but I found the cause: Qt dll conflicts.

When I tried to load source control in VS 2008, Perforce failed to load, without tell anything reason why.

After poking around and found no solutions on Google. I looked at one of the installation notes (sccpluginnotes.txt):

(Bug #14942)
P4SCC.initialization process fails with "Unspecified error" message due
to multiple QT dll conflicts found in the system path.

Workarounds:

Place the Perforce installation directory first in the system path
or rename or move the non-Perforce QT dlls.

So I moved perforce path to the first in the PATH variable, and it worked again!