1.
After the big shootout when Blain got killed, the Predator is wounded and his blood is on at least one leaf. They search the area and find nothing, while Anna finds the blood. How come none of them found blood? They're skilled soldiers, especially Billy with his tracking skills, and they miss a huge green glowing liquid? Anna even wipes it on her clothes right in front of Dillon, and he doesn't notice it.
2.
When Billy laughs at Hawkins' joke, the Predator picks up his laugh. But the laugh at the end of the movie is not the same as the laugh he heard from Billy. This one is a lot longer and doesn't follow the same pattern. Did Predator improvise the laugh or did he just remix what he heard? Did he know what laugh is and how it would sarcastically fit into the context? In that extent, Predators understand sarcasm and humor.
Or it was a lucky guess, the Predator in Predator 2 randomly uses "want some candy?" when it doesn't make sense.