This past Shabbos, the older of my two younger brothers (Hotdog) davened Shacharis for the amud. Now, being that Shavuos had just passed, Hotdog's Tefilah mind was still on the nussach for Shalosh R'galim. He startes Shochen Ad with its incorrect melody. This mistake caused me to glance at the younger of my two younger brothers (Hammy) and snicker about the incident (much like Hotdog and Hammy do to me when I crack while davening for the amud. However, I was a little taken aback when the gabbai walked up to the amud and asked Hotdog to change to a regular Shabbos nussach.
Later on in the Tefilah, my brother once again accidentally switched the nussach. This time, it was by the end of Kedushah. This time, the Rav leaned over and told my brother what nussach he should be doing.
Here is my question: How important is nussach. I am not talking about the words (i.e. words that are different for nussach Ashkenaz and nussach Sefard). I am refering to the tune that the chazzan uses for a particular tefilah. Who decides when to correct it?
If I were to use the Shabbos Mincha nussach during the week would people correct me? I happen to know that the answer to that question is 'no.' I did that in the very same shul with the very same Rabbi and the very same Gabbai. Nobody corrected me. What makes one nussach change more imperative to correct than another? What would have happened if my brother would have continued in that nussach? Would we have been required to duchan? What's the big deal with nussach?