On a current project we're building some sites using the Plumtree portal framework.  To make it all work we need to reference the Plumtree dll's.  In building my projects I would from time to time get a .NET configuration error when building the project.  The error would say:

"access denied" "An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately".

This happened for one of the Plumtree assemblies and show the line in the machine.config "ADD assembly="*".  This didn't happen all the time but was getting to be a headache since I would need to reboot my machine to get my project to compile.

After googling a few times I believe I've found the fix (man I hope so).

CAUSE:
If you run Index Server (Cisvc.exe), then Index Server may rescan the Temporary ASP.NET Files directory while it requests a Microsoft ASP.NET page. Cisvc.exe then holds a lock on the Temporary ASP.NET Files directory for one to five minutes. The length of time of the lock depends on the size of the directory that causes the Aspnet_wp.exeprocess (or W3wp.exe process for applications that run on Microsoft Internet Information Services [IIS] 6.0) to not load the particular DLL.

RESOLUTION:
If you do not use Index Server on the server, you can disable it.  Or if you use Index Server, you can exclude the Temporary ASP.NET Files directory from the folders that the Index Server scans.

Here is the MSDN KB article.