One of the purposes of having a build machine is to eliminate the "Well it works on my machine" problem. The build failed this morning. A developer was using a COM DLL, and had installed from the component-monger's installer, and then copied the DLL to the lib folder and checked it in, which is fine.
The build failed because the DLL wasn't registered on the build machine. Adding the regsvr32 call to the pre-build event in the project meant that the DLL was registered before the project was compiled. The build machine is now completing successfully again.