[kibwen]

Note that C# famously broke backwards compatibility when it added generics. I say "famously" because it opted not to follow Java's type-erasure approach (which was chosen so as to explicitly maintain backwards compatibility), and history seems to agree that the minor amount of pain back then was well worth the improvements that it brought to the language (especially relative to Java).

[pjmlp]

They also changed the for loop var semantics in C# 5.

There a few other little breaking changes as well.