The Broken Seal

is episode 16 of the original Naruto anime.

Plot
Sasuke managed to temporarily hold his own against Haku, an impressive feat in itself, given the fact that Haku was an extremely formidable opponent considering Sasuke's current level. He even awakened his Sharingan, which allowed him to track Haku's incredibly fast movements. Naruto stubbornly refused to accept that Haku was too strong for him, and repeatedly picked himself up to continue to keep hopelessly fighting. Haku threw a volley of needles at a fallen Naruto and, in a split-second decision, Sasuke, fearing the needles to be fatal, jumped in front of Naruto, being struck with them instead. After Naruto witnessed this selfless act of his teammate, he believed Sasuke to have been killed. As he mourned his fallen comrade, he was blinded by immense anger and frustration, and allowed his inner demon to partly overcome him. After this, Haku was severely wounded by an enraged Naruto, who seemed to no longer be affected by the needles that were thrown at him, with this being due to the Nine-Tailed Demon Fox's chakra merely throwing them aside as if they were nothing. Haku, in disbelief, admitted that "this isn't good", before being 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 was shown in the flashback sequence of the dub of this episode.