  Landauer's principle states that the logical irreversibility of an operation,
such as erasing one bit, whatever its physical implementation, necessarily
implies its thermodynamical irreversibility. In this paper, a very simple
counterexample of physical implementation (that uses a two-to-one relation
between logic and thermodynamic states) is given that allows to erase one bit
in a thermodynamical quasistatic manner (i.e. that may tend to be reversible if
slowed down enough).
