The Torah states (Ex. 21:23) that if two people are fighting and accidentally kill an innocent bystander (such as a woman), "you shall give a life for a life."
The Gemara (Sanhedrin 79a-b) records a debate about the meaning of this verse. The Tanna Kamma states that it means that the killer is put to death, since he did intend to kill someone (the person with whom he was fighting). On the other hand, R. Shimon (and Rebbi) say that he need only pay reparations to the victim's heirs, as evidenced by the term ונתתה ("you shall give"), which as in the preceding verse means a financial settlement.*
Now, the same expression is repeated in Lev. 24:18, where it refers to one who kills an animal. There, obviously, it means that he pays for its value (as indeed the Torah itself makes clear three verses later: "one who kills an animal shall pay for it"). That being the case, on what grounds does the Tanna Kamma say that in the first-quoted verse (about killing the bystander) it means execution rather than reparations?
* Actually, Rambam (Hil. Rotze'ach 4:1) understands R. Shimon's opinion to be in accordance with the view of Tanna D'vei Chizkiyah (in that same sugya), that the assailant is liable to neither death nor payment. Tosafos there (ד"ה ומפקא) points out that Tanna D'vei Chizkiyah must therefore necessarily understand the case in Exodus to be talking about where the assailant did intend to kill the woman, and then he is indeed subject to the death penalty; they would exempt him only if indeed he didn't intend to hit her. (Which means that the same question would apply according to this view as well.)