Even in the OF, in a Benedictine monastery (where the liturgy is a bit more elaborate than in your average parish), there are mistakes. More than once I caught out a priest or lector using the reading from the wrong year; once even the priest doing the homily was caught off guard because the acolyte doing the first reading took the wrong one and it had been part of his homily; he had to start off with “well, the reading we were SUPPOSED to hear today…”.
I’ve heard the schola start on a chant with different monks starting a different chant because… the year happened to be one where the chant had to be taken from another day and one or two of the monks were literally, on the wrong page
Then there’s the “A” word during lent " Alle…oops".
Then there’s smart-alec me in my early oblate-in-training years: “hey they’re not supposed to chant the Te Deum during Lent”. Well, true, not on Sundays, but yes, on feasts…
Or: “they took the wrong readings for today…” then realizing, oops… the monastery doesn’t move Ascension to Sunday, they celebrate it on Thursday, that’s why the readings don’t correspond to my Magnificat".
In short we’d better be sure we’re right when we point out mistakes.
I’ll never forget once when at Vigils the monk reading the long bible reading took the reading from the wrong year. I asked the oblate director later “why did Father L. take the reading for year I when we’re in year II?”. The answer caught me a bit off guard: “because Father L made a mistake…”. You mean monks aren’t perfect? No fuss, no muss, just an admission of humanity. Boy that was embarrassing. For me
Personally I’d let it slide, unless it was a whopper and came up regularly. I’ve learned to laugh off the monks’ little bloopers, in fact it makes them feel more human, more approachable.