After encountering alien life for the first time in the film Alien, John Hurt's character Kane is attacked by an organism that attaches to his face (i.e. the facehugger). After attempting to remove the creature unsuccessfully, the crew of the Nostromo decide to let Kane rest. After he awakens, he feels and looks fine, but starts convulsing at breakfast and unfortunately it doesn't have to do with anything he ate (well actually it kinda does). Kane ultimately suffers one of the most horrific and unique deaths in film history in what we now know as the chestburster scene - the alien is birthed from his body, popping through his chest.
After a bevy of sequels and spin-offs in the Alien franchise, this may not seem that unique, but upon the film's original theatrical release the scene shocked, stunned, disgusted, mortified and horrified audiences worldwide. It is still one of the most talked about scenes in film history and although there have been chestbursting aliens to follow, none of them played like this one.