A set of tests from 2008 to demonstrate incorrect ContentPlaceHolder behavior.
This test app is kind of a 'acid test' for two bugs in the .net framework related to ContentPlaceHolder.
Even in .NET 4.0, these bugs persist.
Live test site: http://cph-tests.apphb.com/
Article explaining background: http://www.nathanaeljones.com/blog/2008/referencing-stylesheets-scripts-from-content-pages