Kiddushin, Chapter Two, Mishnah Two. This mishnah deals with cases in which a woman assumed that the item with which the man was betrothing her was a particular item - for example, when it was dark and she could not see - and afterward it turns out that it is not the item she had in mind and agreed to.
The cases in the mishnah:
"Hiskadshi li b'kos zeh shel yayin" - "Be betrothed to me with this cup of wine," and it turns out to be a cup of honey, and likewise the reverse: he said "shel devash" - "of honey," and it turns out to be wine. Either way, this is not what she assumed and not what she agreed to.
"B'dinar zeh shel kesef" - "With this silver dinar," and it turns out to be gold, meaning that the coin he gave was gold, which is worth more; and likewise the reverse: he said "shel zahav" - "of gold," and it turns out to be silver.
"Al menas she'ani ashir" - "On condition that I am wealthy," and it turns out that he is poor, and likewise the reverse: he said "Al menas she'ani ani" - "On condition that I am poor," and it turns out that he is wealthy.
In all of these cases the mishnah rules: "Einah mekudeshes" - she is not betrothed, since this is not what she agreed to. And even though at first glance one of the items seems superior to the other, that is not necessarily so: even marriage to a poor man may sometimes have an advantage over marriage to a wealthy man, and it all depends on the circumstances.
The view of Rabbi Shimon:
Rabbi Shimon says: "Im hitah l'shevach - harei zu mekudeshes" - if he misled her in a way that turned out to her advantage, she is betrothed. That is, if he misled her regarding something that is better and more valuable than what he said, she is betrothed.
The Gemara explains that Rabbi Shimon's disagreement with the first opinion does not apply in every situation. When the woman herself receives the betrothal, even Rabbi Shimon agrees that even if the item appears to be superior, she is not betrothed. The only case in which he disagrees is where the woman appointed an agent to receive her betrothal, and told him: go and receive the betrothal on my behalf from so-and-so, who said that he is betrothing me with a silver dinar - and the agent went and received a gold dinar.
According to the first opinion: she is not betrothed, since she wanted to receive specifically a silver dinar and not a gold dinar.
According to Rabbi Shimon: she is betrothed, since in saying "a silver dinar" she was not being precise in her wording but was merely giving the agent a point of reference - she was simply passing on to him the information she had been told, and she would certainly agree if he were to receive a more valuable item.
It emerges that when a woman conveys instructions to an agent, she is not precise in her wording but is merely passing along information; but when she stands directly opposite the man betrothing her, the betrothal must be exactly as he stated.
In summary: in this mishnah we learned that a betrothal made in error - whether regarding the type of item (wine and honey, silver and gold) or regarding the status of the man betrothing her (wealthy and poor) - does not take effect, even when the error is seemingly to her advantage, since what she received is not what she agreed to. Rabbi Shimon holds that "if he misled her in a way that turned out to her advantage, she is betrothed," and his disagreement applies specifically in a case where the woman appointed an agent to receive her betrothal, where her wording is considered merely a point of reference.