The Gemara says that if someone made a Neder not to eat today, although really he should be permitted at night, However the Rabanan required him to eat only with Hataras Nedarim. R' Yosef explains: its' a gezaira so he shouldn't permit to eat at night when he makes a Neder for a day, which he needs to refrain fro 24 hours.
R' Yirmiya says it's a Kanos (fine) for making a Neder,since he did wrong by making a Neder in the first place.
Ran explains: although regularly we don't fine people to extend their Neder to fine them. However, here there is a reason to extend it because of R' Yosef's Gezaira. However, R' Yirmiya holds that in itself it's not enough to worry that he'll become confused between the two Nedarim. However, since he did wrong for making the Neder we'll impose this gezaira on him and make him to make sure there is no chance whatsoever to mix them up.
