I have an online automatic stock purchasing account which purchased stock for me on Yom Tov (by accident! even though i put in the order the week before, I forgot that day was going to be Yom Tov). What do I do now? Is it assur to keep the stocks? Did I do an issur?
Although it would not be correct to intentionally make an order for purchasing stock on Shabbos or Yom Tov, post factum there is no problem in keeping the stock, and this would not be considered an issur, certainly if done without intent.
Sources: Authorities dispute whether it is permitted to arrange for a kinyan to take place on Shabbos. Rabbi Akiva Eiger (159) cites a proof from Terumas Hadeshen (269) that there is no prohibition is doing so, but continues to prove that it is prohibited. Iggros Moshe (Orach Chaim III, no. 44) defers both proofs, and writes that one should be stringent, and this is also the opinion of Radvaz 456. However, a number of authorities are lenient in this matter (see Shaul Umeishiv VI, no. 50, and Divrei Shaul, Shabbos Hagadol, concerning selling chametz on Erev Pesach that falls on Shabbos; Maharshag 13; Maharam Schick 131; Machaneh Chaim, Orach Chaim III, no. 22), and see also Avnei Nezer (Orach Chaim 51).
Thus although in deference to stringent authorities, it is not proper to intentionally arrange a sale for Shabbos, as a rabbinic prohibition (making sales on Shabbos is a rabbinic prohibition) in doubt, it cannot be called a transgression. Certainly, if done without intention, no issur can be said to have been transgressed (according to Nesivos Hamishpat, even a certain rabbinic prohibition transgressed without intent is not considered to be a transgression), and no repentence is required.