My son lives in Givat Ze'ev. Following the Feinstein shitta regarding a Shemitta esrog, he had to acquire his at the pardes, not from a merchant. After a great deal of effort, he did locate a cooperating pardes, and he found a beautiful esrog.
Another shitta of the Feinsteins is that the Shaas HaBiyur of esrogim is Rosh HaShannah. So he put his esrog into a bag and put it down in the street in front of his house, and in the presence of three adults he said he was mafkir it, that anyone that wanted could take it. So he was mekayeim the requirement of biyur.
His nine year old daughter was watching. She said, Tatti, what do you mean? My son explained what hefker means. Miriam asked, does that really mean that anyone can take it? Yes, my son said. So dear Miriam grabbed it and said "Now it's mine."
Short of the bag being run over by a truck, this is the worst thing that could have happened. Sukka 46b-
א"ר זירא לא ליקני איניש הושענא לינוקא ביומא טבא קמא מ"ט דינוקא מקנא קני אקנויי לא מקני ואשתכח דקא נפיק בלולב שאינו שלו
My father in law presented a bigger problem. The reason מציאת קטן לאביו is because מריצה אצל אביו. But in this case, Miriam took it davka not to give it to her father. She asked her father, what do you mean, he said, "anyone can take it," she said "Good! Now it's mine!" You can't have less of a מריצה אצל אביו than that. It is the opposite of מריצה אצל אביו.
My nephew, Rav Gilan Grant, argued that מריצה אצל אביו is only one tzad in the Gemara, and the alternative is משום איבה, which probably would apply here. In Shulchan Aruch, only the svara of משום איבה is brought down, so the מריצה אצל אביו problem is solved. Although one might argue that even איבה would not apply where the father was mafkir, it is likely that the svara of איבה creates a universal din, applicable even where there is no real issue of creating איבה.
Someone suggested that my son marry her off to a friend, so her property becomes the husband's, and, as the husband is an adult, he could give the esrog back to my son. We're hoping it won't come to that. Anyway, the husband's kinyan on his wife's property is most likely miderabanan, and nobody wants to rely on a kinyan derabanan for a mitzva deoraysa.
R D Eli was bothered by the whole hefker business, since an Esrog is less than מזון ג' סעודות. I answered that from Tosfos in Rosh HaShanna there's a mashma'us that an esrog is called more than מזון ג' סעודות, but the truth is that we were machmir to be mafkir because Reb Moshe in his Teshuva in OC 1:186 says to be mafkir. We figured he says so because the person that took the Esrogim in the first place took more than מזון ג' סעודות. But to tell the truth, he shouldn't have been mafkir. It was a mistake, and some of the local poskim added the issue of hefker ta'us to the mix.
UPDATE: Motza'ei Yom Kippur, we listened to the message Shlomo left us after Yom Kippur was over in Israel. Among other things, he let us know that the infamous esrog turned out to be passul- it is chaseir.