My favourite word at the moment is "disingenuous". It's much less direct, but still as effective as saying simply that someone is a liar: "It's disingenuous to suggest that I neglected to perform that duty".

Very nice in company emails. Will have most people rushing for the dictionary.

I subscribe to the same theory of wordplay that The Police did in their song "De Do Do Do, De Da Da Da": "when their eloquence escapes me, their logic ties me up and rapes me"; I love tying people in knots with vocabulary.