It was Parshas Vayakhel Pekudei a few years back. I was laining in the parsha of the me’il. In Parshas Tetzave, the word safah has a pazeir. In Pekudei there is a meircha-tvir on safah lefiv saviv lo yikare’a. So as I’m about to lain the much softer pekudei trop, the guy with the aliya standing right next to me goes safa-a-a-a-a-h with a pazeir, quite looud enough to throw me for a moment. So I let my better judgement get away and followed his lead. There’s only 5 words till the sof pasuk! No way to recover and blend in such short order! Ended up with egg all on my face. Serves me right for not having enough self confidence to ignore him!!
The mishna berura says that whenever one davens, even if he davens softly, he must hear the words coming out of his month. (Tzarich lehashmia l’aznav ma shemotzi b’sfasav.) The only exception the mishna berura lists, is that when one has an aliya and is reading along with the Baal Kriah, he may say so soft that he cannot hear, so as not to disturb the Baal Kriah.