verb
to mix apples and oranges
“Era cierto, mi educación católica pesaba mucho, y si algo iba en contra de lo que me habían inculcado en casa, rezaba como una devota, pero no hay que mezclar churras y merinas.” — It was true, my Catholic education weighed heavily, and if something went against what had been instilled in me at home, I prayed like a devotee, but there is no need to mix apples and oranges.
Literally, “mix churras and merinos”. Both churras and merinas are Iberian sheep breeds.