Nedarim, Chapter Three, Mishnah Three
Our mishnah begins to discuss vows broken under pressure. There are actually two types of such vows. Our mishnah discusses the first type, whereby a person made a fully valid vow, but then due to circumstances beyond his control, was not able to keep the vow.
Vows [broken] under pressure: if one subjected his neighbor to a vow to eat with him, and then he or his son fell sick, or a river prevented him [from coming] such is a vow [broken] under pressure.
In this case Reuven says to Shimon that Reuvens things will be forbidden to Shimon if Shimon does not come over to eat at his house. Shimon intends to come but cannot due to his or his sons sickness, or his inability to cross the river. The mishnah rules that when Reuven made the vow he did not intend for it to be valid if Shimon wanted to come but was prevented by circumstances beyond his control. His only intention was that his things should be prohibited to Shimon if Shimon did not come because he chose not to. Therefore, the vow is invalid.