Daf Yomi - Yoma 46 - Going Beyond the Mitzva

The Mishna had previously stated that every day they would bring a p'ras of ketores in the morning and afternoon. It would seem that this was done on Shabbos, as well. Gevuros Ari asks that according to Rashi in Zevachim (109) that holds midoraysa, a kezayis of ketores would be sufficient, why is it permissible to burn the entire amount - the extra should be considered a burning for no purpose and should not be allowed?

The Acharonim answer this question based on a Netziv that holds that once a person begins doing a mitzva and in the process he does more than he is required of doing, all the extra is considered to be part of the mitzva.

There are many examples to this yesod. Care to share one with us?