Breaking changes in .NET 4.0

Question

The languages documentation team publishes separate documents for C# and VB breaking changes:

VB: http://msdn.microsoft.com/en-us/library/cc714070%28VS.100%29.aspx

C#: http://msdn.microsoft.com/en-us/library/ee855831%28VS.100%29.aspx

I wrote the C# one and included covariance and contravariance breaking changes mentioned by Eric Lippert, and events changes discussed by Chris Burrows. There are also some breaking changes around optional parameters, embedded interop types, and method group type inference.

Update:

One more useful document (from .NET documentation team): http://msdn.microsoft.com/en-us/library/ee941656%28VS.100%29.aspx

Covariant and contravariant conversions introduce some obscure but possible breaking changes upon recompilation:

http://blogs.msdn.com/ericlippert/archive/2007/11/02/covariance-and-contravariance-in-c-part-nine-breaking-changes.aspx

The C# user education team compiles a list of the known potential breaking changes in the new version of the compiler, see the answer above for details.

Also note that SmtpClient now implements IDisposable so you should now use something like this:

using (var smtpclient = new SmtpClient())
{
  smtpclient.Send(message);
} 

According to this page there might be more of these 'hidden jams' inside the .NET 4.0 framework. And the author suggest to find them by using FxCop on your code.

Just to add to the list, here's the ASP.Net official list for the RC, I haven't come across an RTM one yet.