Fish scales were found in the body of the victim, meaning the bullet must have traveled through the fish before entering the body of the victim, but the bullet was found IN the fish. Had the bullet passed through the victim and ended in the fish (as they theorized), no scales would've been present in the victim's wound.
When Danny is taking a photo of the dead body tell Steve to move on get out of the way. Even though there is very little space on the ridge, when Kono pulls up the photo later Steve is nowhere to be found.
After Steve McGarrett breaks his arm and begins to observe his body body, he takes out his iPhone which displays the following:
'No Service', '4:25 PM', '92% Battery'.
Later we're told they found the body in the morning. Clearly 4:25PM is not morning.
When Dano is picking up the fish to find one with a bullet in, he is wearing gloves. Steve removes the bullet and puts in Dano's gloved hand. Then Dano takes the glove off with the bullet in it. So Dano's hand didn't touch the fish at all. So when Dano wiped his hand on Steve's shirt it wouldn't have smelt like fish. Later Kono complains that Steve stinks. Another mistake.
When on the mountain and Steve wants to climb back up to where Danny is, he tells Danny to take up the slack on the rope. Danny simply pulls the rope up, hand over hand. This is completely wrong, as there is no anchoring if anything happens, and the rope will slide back down - which is exactly what happens. At the least, Danny should have had the rope around him.
Danno reveals that he is a Yankee fan at the start of the show, but in a later one he claims that he was a Met fan, so much so that he knew the entire roster of teams from the 80s and 90s.