We all make mistakes ... I know I certainly do. Today I made one that only set me back about 10 minutes ... but I admit I felt silly after I realized what I did wrong. Today I created a new VS 2008 project, added an Entity Data model (aka EDMX file) to the project, and began setting up my EDM. When i went to build the project I received a ton of errors about invalid references. So I checked my references folder and bam! ... several dll's were marked is invalid.
The quick and easy solution to this problem was that I forgot to set the target framework of my project to .NET Framework 3.5.
Once this is done everything is happy as Ferris Bueller in a parade.