The Mechaber paskens in S.A. that if your tallis falls off completely then you make a new bracha when putting it back on. The present Chief Sefardic Rabbi, Rav Dovid Yosef, paskens in his [incredible!!] Halacha Brura [volume 1 of 184] that one should NOT make a bracha [despite his father Chacham Ovadiah's (מופת הדור ממש) insistence, in most but not all cases, to follow the Mechaber]!
I mentioned this in shul and someone commented that R' Ben Tzion Abba Shaul says that if your tallis falls off towards the end of davening - don't put it back on to avoid all sfeikos regarding the bracha.
This is a short comment that can and should be expanded.
