@davidho this is a typical Parisian trap… the t+ tickets are only available “intramuros" (so only for some zones which cover Paris itself and “some" outside stations) and the gates only know where you enter, not where you intend to get out so can't enforce that you use the proper ticket… The quintessential example for this is La Défense which you can reach either by RER (with a more costly ticket) or via the Métro with a t+ ticket… 😬