The Broken Seal

is episode 16 of the original Naruto anime.

Plot
Sasuke temporarily manages to hold his own against Haku, an impressive feat in itself, given the fact that Haku is an extremely formidable opponent at Sasuke's current level. He even awakens his Sharingan, allowing him to track Haku's incredibly fast movements. Naruto stubbornly refuses to accept that Haku is too strong for him, thus, he repeatedly picks himself up and continues hopelessly fighting. Haku throws needles at Naruto and in a split-second decision, Sasuke, fearing the needles to be fatal, jumps in front of Naruto and is struck with them instead. After Naruto witnesses this selfless act by his fellow teammate, believes Sasuke has been killed. As he cries over his fallen comrade, he is blinded by immense anger and frustration and allows his inner demon to partly overtake him. After this, Haku is severely wounded by an enraged Naruto, who does not seem to be affected by the needles he throws at him. This being due to Demon Fox's chakra merely shunning them aside as if they were nothing. Haku, in disbelief, admits that "This isn't good", before getting punched out of his Ice Mirrors.

Trivia

 * Although edited out of the English dub of Sasuke and Sakura: Friends or Foes?, the scene of Naruto accidentally kissing Sasuke is shown in the flashback sequence of the dub of this episode.