It's pretty well known that you can't fulfill a mitzvah with something that has been stolen. However, what if you buy or find something to fulfill a mitzvah that (unbeknownst to you) had been stolen by the person who sold or lost it?
Ari steals a lulav and sells it to Benyamin. On Sukkot, Benyamin takes lulav, but later learns that his lulav was stolen. Has he fulfilled his obligation that day, or must he re-take with another lulav?
Ari steals a lulav and loses it. Benyamin finds the lulav, tries to find its rightful owner, but can't. On Sukkot, Benyamin takes lulav with his found lulav, after which Ari confesses to him that he had stolen the lulav in the first place. Has Benyamin fulfilled his obligation to take lulav that day?