Thurs Nazir 33

Abaya held the case of Bais Shammai that a mistaken Hekdish takes effect, if he made it after the day is over, and he is Mikadesh what he thinks comes out first.

Tosfos explains: then it's possible to gauge what he thinks come out first, because he recognized the footprints of the ox that came out first or that you saw which came out first an then forgot. However, if you make a guess on the future what will come out, it's not similar to Temurah where there is a reason to make a mistake, since you have not the slightest inkling which ox will leave first, for you're not prophet.