Fixed-points (or fixpoints)
The idea of defining the semantics of lambda calculus by reducing first every expression to normal form (for which a simple mathematical denotation exists) by a sequence of contractions is attractive but, unfortunately, does not work as simply as suggested… The problem is that, since every contraction step … removes a ?, we have deduced a bit hastily that it decreases the overall number of ?s. We have neglected the possibility for a contraction step actually to add one ?, or even more, while it removes another. – Meyer