There is also the matter of some of the station names being so long
and similar that they get abbreviated on signs, in ways that may not
be obvious to foreigners. I don't remember any real examples offhand,
but it wouldn't surprise me to see "Montreuil" used instead of "Mairie
de Montreuil" to mean eastbound on line 9, say. One might easily think
that was a different station, maybe on a different line; and an
English-speaker might also think that "Mairie" was the important word,
since it comes first, and would never be omitted in abbreviating.

That one can go one worse - if there's a definite article involved.
Mairie des Lilas station is in (and is often shortened to) Les Lilas.