I remember well years ago, not so long after I had made aliyah, that a new oleh moved into the neighborhood. Not just a stam oleh, but a talmid hacham (and I don't use that term lightly).
Well, the practice in our shul, a widespread practice all over Israel, was for the shta"tz to lower the volume of his voice as he prompted the cohanim with birkat cohanim, word by word.
The new oleh was not fond of this practice. Why? Well, in all honesty, the poskim, even the rishonim, talk about this practice, and it's pretty unarguable that their directives call for the sha"tz to remain at the same volume of voice. The new oleh made his discomfort well known, which brought to the shul some degree of discord.
Why discord. It seems pretty reasonable to oppose something the rishonim and poskim opposed, right? I mean, there it was in black and white, just staring at us in the face. And shouldn't our behavior follow the written Halakha?
Let's see how the tosafot may have dealt with this issue.
In the first mishna of the 7th chapter of Berakhot, we learn:
שלשה שאכלו כאחת חייבין לזמן.
Three people who eat together must say zimun.
The gemara brings a disagreement between Rav and R. Yochanan as to whether two men are allowed to make a zimmun.
To answer that question, the Gemara brings the following beraita from Arachin:
תלמוד בבלי מסכת ברכות דף מה עמוד ב
תא שמע: נשים מזמנות לעצמן, ועבדים מזמנים לעצמן, נשים ועבדים וקטנים אם רצו לזמן - אין מזמנין; (והא נשים אפילו מאה) והא מאה נשי כתרי גברי דמיין, וקתני: נשים מזמנות לעצמן ועבדים מזמנין לעצמן! - שאני התם, דאיכא דעות.
That is, since the din of two men is equal to the din of a hundred women, and since we learn in a beratia that women do make a zimmun, then two men should be able to make a zimun. But the Gemara rejects this argument as follows: while it is true that the din of two men is generally equal to the din of three women, in this case it doesn't apply, since … it is critical to have three unique individuals for zimmun.
Now, take a look at Tosafot (patiently … we will get to our issue shortly).
שאני התם דאיכא דעות - מכאן משמע דנשים יכולות לזמן לעצמן וכן עשו בנות רבינו אברהם חמיו של רבינו יהודה ע"פ אביהן ומיהו לא נהגו העולם כן וקשה אמאי לא נהגו מדקתני מזמנות משמע דקאמר חייבות לזמן וי"ל דנשים מזמנות לעצמן היינו אם רצו לזמן מזמנות וכן משמע קצת הלשון מדקתני בסמוך נשים ועבדים אם רצו לזמן אין מזמנין ועוד דמדמה ליה הגמרא לשנים משמע דחובה ליכא והא דקאמר בריש ערכין (דף ג. ושם) הכל מחוייבין בזימון לאתויי נשים לענין רשות קאמר ולא לענין חובה
Tosafot say that from this suggya, it is apparent that three women can make their own zimmun, and, in fact, that's exactly what the daughters of R. Avraham did. Just one problem … tosafot notes that this is not widely practiced. They looked around and saw that women are not making zimmun. Why not? Tosafot concludes that when the Gemara says "women make zimmun" it must have meant they have the option to make zimmun, but not that it is obligatory. But there is a slight problem with this conclusion, note the tosafot. The Gemara in Arachain 3a writes that women are "obligated" to make zimmun.
Now we have a problem. Tosafot note that women are not making zimmun. But the Gemara in Arachin specifically says they are obligated. It's right there, in black and white!
So how do Tosafot resolve this problem … and I kid you not: Nah, that's not what the Gemara in Arachin means. When it says that women are "obligated", it really means that it's "optional".
In the immortal words of Maxwell Smart, "Would you believe." (Google it if you're too young to understand the reference).
So, for those of us who think that our behavior must always follow the written Halakha and never the other way around … guess again. The re-reading of texts to fit common practice is a well known phenomenon among Ashkenazic rishonim. This particular instance is just the most classic case that comes to mind.
As a side note, the development of the minhag whereby the sha"tz lowers the volume of his voice when prompting the cohanim is fascinating. The Yesodei Yeshurun notes that there was a difference between Bavel and Israel. In Bavel, the custom was to prompt, and in Israel not to prompt (based on the prohibition of a non-cohain to do birkat cohanim). I suggest that the minhag for the sha"tz to lower his voice developed as a comprise between the two postions of yes prompting versus no prompting, i.e., the sha"tz lowers his voice significantly when prompting. That a minhag takes the middle road between two approaches in Halakha, adopting neither one of them, is a common phenomenon. Another example is with the placement of mezuzah. There are two approaches in Halakha: vertical and horizontal. A commonly practiced minhag is to place it on a diagonal, as a comprise between the two approaches. This phenomenon is discussed in Sperber's Minhagei Yisrael. The application of it to this particular issue is my own chiddush, which I believe to be correct, but take it for what it's worth.