We had an interesting error this morning.  A client called to say that they had upgraded a bit of software (a web application hosted on their SBS 2003 server), and were now getting a .NET error:

Could not load file or assembly ‘System.EnterpriseServices.Wrapper.dll’

After some searching around, we managed to fix the error by copying the %systemroot%\Microsoft.NET\Framework\v2.0.50727\System.EnterpriseServices.dll [...]