The Mordechai holds (see OC siman 13, MG"A) that a person is allowed to wear a 4 cornered garment on shabbos even if it is missing tzitzis. Since one is not allowed to tie tzitzis on shabbos, there is a ptur ones which exempts one from the mitzvah.
(The Mordechai seems to hold that ones is not a maaseh aveira with a ptur from onshim, but is not a maaseh aveira at all.)
Rav Noson Gestetner (in Maor haTorah) asks: the SA paskens (YD 120:16) that if you forgot to be toveil a kli before shabbos you should be makneh it to an aku"m and then borrow it back to use. According to the Mordechai, why is this necessary? Since you cannot be toveil a kli on shabbos, you should be exempt from the mitzvah based on the ptur of ones. Why is tevilas kelim any different than the case of tzitzis?
This is such a nice question that I'm going to leave it up as-is without answers or comments so it has time to marinate. Sometimes you need to pause to savor the "im tomar" before rushing to the "yesh lomar." Bli neder will come back to it.