compiler build error : The call is ambiguous between the following methods or properties

Took me a while to figure this one out but Gluips comment is the right one, I'll add it here for easy reference:

That's it! Somehow the project was directly referencing itself. Removing the reference probably fixes the problem. Thanks!

This fixed it for me.

Ok having done a bit of playing round I can reproduce a similar situation to this by adding a file reference to the output file - the first time round it builds successflly (as the reference is not used - you simply get a "reference not resolved" warning), however from this point on I see the "The call is ambiguous" error appear in the code editor.

For me however this doesn't prevent me from building the solution (I'm testing this using Visual Studio 2010), however the error does appear - maybe under slighly different circumstances, such as a different Visual Studio version, this would stop the project from compiling.

You could also engineer this same situation with post-build steps that copy the output assembly.