Plot hole: McGarrett is hunting for heroin dealers who've laced the drug with arsenic. Yet when he and Chin Ho arrest Harry Parch and find his stash, McGarrett performs the TV cop cliché of sticking his finger into the white powder and tasting it. Even that small a dose of arsenic could be lethal, and seasoned cop that he is, McGarrett would know that.
Continuity mistake: The HPD snipers watch McGarrett's office window, trying to get a clear shot at Collins. In camera views from inside the office, there's a coat rack with a hanger on it standing in front of the window. But in all the shots from the snipers' POV, the coat rack isn't there.
Continuity mistake: The bomber wants revenge on Danno for killing his brother, Joey Collins, whose death is recapped in flashbacks to a first season episode in which Danno first killed someone in the line of duty. But in that episode, Joey's name wasn't Joey or Collins - it was Thad Vaughn.