Matching is a solving process which is crucial in declarative (rule-based) programming languages. In order to apply rules, one has to match the left-hand side of a rule with the term to be rewritten. In several declarative programming languages, programs involve operators that may also satisfy some structural axioms. Therefore, their evaluation mechanism must implement powerful matching algorithms working modulo equational theories. In this paper, we show the existence of an equational theory where matching is decidable (resp. finitary) but matching in presence of additional (free) operators is undecidable (resp. infinitary). The interest of this result is to easily prove the existence of a frontier between matching and matching with free operators.