You buy a new Talis and after you wear it you notice a defect. You want to return it to the store for a refund or replacement. However a person is only obligated in Tzitzis if he owns or borrows the garment. Now if you return it does this make the Bracha you made on the Talis a Bracha L'Vatala?
You buy an expensive Esrog. During Succos it starts going bad. If you demand your money back, what happens to the Mitzva you did on the first day of Sukkos? On the first day you are only obligated if the Esrog belongs to you. If you cancel the sale, then the Esrog did not belong to you.
Possible practical ramifications:
Perhaps one should not return the item in such a case.
Perhaps once one has decided to return the item, one should cease making a Bracha over it.
How does a person deal with these type of cases?