The ending of Croc vs. Shark, when the crocodile has to come up to the surface for air after a two-minute confrontation, allowing the shark to sneak up on it and tear out its underbelly. Real crocodiles can hold their breath for up to half an hour. Also, the shark is inexplicably still able to swim perfectly fine after the crocodile tears off one of its pectoral fins.
It's safe to say the outcome of Polar Bear vs. Walrus wasn't foreseen by most.
Elephant vs Rhino had a bit of it with "The rhino couldn't reach the elephant's neck, even with a four-foot horn", as we watch the rhino raise its head up enough to reach the elephant's throat and then some when they're face-to-face.
Wolf vs. Cougar has the wolf being killed by the cougar cutting its leg and throwing it to the side. It's not even a gradual bleeding to death, the wolf just dies instantly from a minor cut. Although it was implied that the wolf was not actually dead, but merely knocked out. The cougar ran away before it could finish the job because it heard the rest of the wolfpack approaching.
Jaguar vs. Anaconda has one when the jaguar gets distracted by a bird call, allowing the anaconda to sneak up on it and coil around it (not lethally).
Fight Scene Failure: Croc vs. Shark, Lion vs. Tiger and Bear vs. Gator are widely considered low points in this regard.
Awesome Music: The nu metal soundtrack by Trevor Coleman will get you pumped for the fights.
The ending of Polar Bear vs Walrus was EPIC. It actually makes watching the other subpar episodes preceding it worthwhile.
Cougar vs Wolf, if only because it's one of the few times when the big cats win.
Narm: How does the gorilla kill the leopard? By chopping the latter’s back with its arm...and shown via a dramatic spinning shot no less, thus making it look like the ending of a video game fight. You almost expect a deep-voiced narrator to shout, “Leopard chop! Gorilla wins!”.
Nightmare Fuel: Some of the deaths in fights are definitely this; for instance Hippo vs Bull shark has the hippo biting the shark's head, with the narrator saying the shark is Impaled with Extreme Prejudice by the hippo's tusks and crushed and Elephant vs Rhino which gives the rhino's point of view as the elephant rears up and crushes it.
Jaguar vs. Anaconda qualifies due to the suspicious coincidences surrounding the anaconda's "victory".
Lion vs. Croc isn't bad by itself, but placed side-by-side with Bear vs. Gator, it's inexcusable. Why was the bear able to pull off what the lion couldn't, especially when the odds favored the gator much more heavily in that fight than in Croc vs. Lion? On the one hand, the croc was much larger and heavier than the gator, and the bear had the dextrous anatomy of...well, a bear. On the other, this actually works more in favor of the alligator getting poor representation—the series in general seems to give crocodilians (and strict predators in general) the Butt-Monkey treatment, favoring the herbivores and omnivores more.
Special Effects Failure: Ignoring the pretty mediocre CGI, the lion's whiskers can clearly be seen merging into the dead tiger.
Tear Jerker: After the gorilla kills the leopard in Gorilla vs. Leopard, viewers might be worried about the leopard's now orphaned cubs she was defending.
