Gittin 83 - No Strings Attached to the Get!
Rabbi Eliezer validated a Get with a condition "You are permitted to marry any man but so-and-so." Rabbi Eliezer’s learning was supreme, but after his death many Rabbis got together to disprove this teaching of his. However, all but one of their rebuttals had potential flaws.
The undisputable proof came from Rabbi Elazar ben Azaria. He deduced the requirement of complete severance from the term that the Torah used, "Kritut," as opposed to a possible shorter word "Karet." This, according to Rabbi Elazar Ben Azaria, teaches that temporary conditions in the Get are valid, but permanent are not.