Question: I have a Visual Studio 2010 solution with a few projects. I can successfully build with Build -> Build Solution or Build -> Build MyCompany.MyProduct.MyProject. After building I press CTRL+F5 and successfully run the start up project. This works as expected.

After I change a source file, save it and then again build (with either Build -> Build Solution or Build -> Build MyCompany.MyProduct.MyProject) it builds successfully. I am still able to press CTRL+F5 and run the startup project. This again works as expected.

The problem is in the following scenario:
After I change a source file, save it and directly press CTRL+F5 then Visual Studio 2010 says BUILD FAILED and there are no errors. And it doesn't start my startup project (of course).

Am I missing something?

Answer: 
Close Visual Studio, delete the `.suo` file for your solution. It's a 'hidden file' in Windows and contains user preferences, but it can also do nasty things.
Seems a bit late, but what worked for me was to check my Output window.

I had a reference to a Function that didn't exist yet. I don't know why it never came up in the Error List, but the Output window told me exactly where the problem was.