During my first implementation extending the Java collection framework, I was quite surprised to see that the collection interface contains methods declared as optional. The implementer is expected to throw UnsupportedOperationExceptions if unsupported. This immediately struck me as a poor API design choice.
After reading much of Joshua Bloch's excellent "Effective Java" book, and later learning he may be responsible for these decisions. I would think declaring two interfaces: Collection, and MutableCollection which extends Collection with the "optional" methods would have led to much more maintainable client code.
There's an excellent summary of the issues here.
Was there a good reason why optional methods were chosen instead of the two interfaces implementation?