After reading the above posts I remained unsatisfied because of what I see as a restriction of the scope of the question.
So let me explain how I would like to expose it again.
First of all, it's clear that the English
quid pro quo and the French
quiproquo are mutually false friends: briefly summarized, the first one talks about actions (exchanging things, mutual behaviour, and so on) while the latter talks about situations (confusion between two persons).
This was widely commented in the posts I quoted, and it's ok.
But the precise question "What should be a good equivalent for the French
quiproquo?" didn't get a real and complete answer.
In fact all posts have only took in account its sense of mistaken identity, but implicitly talkin about people only.
I agree this is the true primary sense, directly due to the litteral translation from latin:
qui stands for a person (BTW here we can notice the logical consistency:
quid stands for a thing, hence the different sense for the whole formula in English).
But here is the point: in today's current French, this first sense has been widened, so it now concerns not solely persons but also events or even things, in somewhat unclear limits.
Here are some examples:
primary sense, about persons
-On m'a dit de m'adresser à Mr Dupont. C'est bien vous ?
-Oui mais Dupond avec un "D" : je pense que vous voulez parler à l'autre Mr Dupont, avec un "T".
-Excusez-moi, c'est un quiproquo.
-On ne t'a pas vu à la réunion hier.
-On m'avait dit que c'était demain : il y a eu un quiproquo !
-J'ai allumé le chauffe-eau mais il n'y a pas d'eau chaude à la douche ! Il y en a pourtant au lavabo.
-Il y a un chauffe-eau séparé pour la douche.
-Ah ! Si on m'avait prévenu il n'y aurait pas eu ce quiproquo.
As you can see, in French
quiproquo is essentially matter of ambiguity leading to a mistake, whatever it concerns.
NOTE: French readers might criticize my comment, noting that the expansion of the concept of the person to "everything and anything" is at fault. True, but it is equally true that it is the current use!
So, again the question: is there an English equivalent which would cover this entire scope?