The mishna in Bava Kama 3:11 brings a case:
היו שניים רודפין אחר אחד, זה אומר שורך הזיק, וזה אומר שורך הזיק--שניהם פטורין; ואם היו שניהם של איש אחד, חייבין
Two [oxen] were pursuing one [ox, and the pursued one was gored]; [the owner of] this one says "Your ox did the damage" and [the owner of] the other one says "Your ox did the damage" - both of them are exempt [from paying damages]; but if both belong to one person, he is liable.
The gemara explains that in the latter case, the owner only has to pay up to the value of the less valuable animal. So far so good. Ready for the next part of the mishna?
היה אחד גדול ואחד קטן, הניזק אומר גדול הזיק, והמזיק אומר לא כי, אלא קטן הזיק... המוציא מחברו, עליו הראיה
If one [ox] was big and the other small, and the plaintiff claims that the larger ox did the damage while the defendant claims that the smaller ox did the damage, the burden of proof rests on the plaintiff.
Understandable at face value, right? But then Kehati, paraphrasing the Bartenura, explains that this actually means that if the plaintiff cannot prove his claim, then the defendant doesn't even have to pay the lesser value!
Is this not a direct contradiction to the first part of the mishna, i.e. "if both belong to one person, he is liable"?