You installed the webapplication project update or installed VS2005 SP1, and you developed your web application using with that kind of project.
Now you want to build it on a build server using MSBuild. If you didn't install VS2005 with that specific update also the build machine, the first time you try to run it you get the following error:
"The imported project "C:\Program Files\MSBuild\Microsoft\VisualStudio\v8.0\WebApplications\Microsoft.WebApplication.targets" was not found"
The solution for that problem is very easy:
just copy from you machine the Microsoft.WebApplication.targets file and put it inside "C:\Program Files\MSBuild\Microsoft\VisualStudio\v8.0\WebApplications\". Probably you have to create also all the folders in that path.
It happens to me all the time I set up a new build server, and always forget about it 