The most recent episode of Marvel's Agents of SHIELD, "Farewell, Cruel World,' went all the way back to the beginning with a reference to Agent Phil Coulson's death in Marvel's The Avengers.
On the current season of Agents of SHIELD, Phil Coulson and the rest of his team are trapped in a virtual reality called the Framework. They've all been given new and different lives to live with no memory of where they came from, but the memory work done on Coulson as part of his recovery in TAHITI allows him to have brief flashes of what he was before.
In "Farewell, Cruel World," Coulson and the team find a back door out of the Framework. Coulson walks towards it but is shot several times. He lives and Agent May helps in towards the back door portal, but not before looking at the blood on his chest and noting that it feels "familiar," a reference back to his death in Marvel's The Avengers.
Fans of the Marvel Cinematic Universe will remember that Phil Coulson was once just the charming agent that Nick Fury and SHIELD would send to deal with the likes of Tony Stark or to investigate a mysterious hammer that fell in New Mexico.
In Marvel's The Avengers, uniting Earth's mightiest heroes was proving to be more difficult than Nick Fury may have though. That all changed when Loki's forces attacked SHIELD's helicarrier. During the battle, Coulson attempted to stop Loki and ended up with a bladed scepter through his heart as his reward. It was a sad moment, but Coulson's death was at least with purpose since it helped unify the Avengers against Loki.
This moment was vital to the start of Marvel's Agents of SHIELD. The television series brought Coulson back from the dead. The explanation was that Coulson had a near death experience but did survive and was sent on vacation to Tahiti to recover. However, it was plain from the start that there was more to it than SHIELD was letting Coulson know. The resurrection of Phil Coulson and the reasons for his missing memories would be a major plot point in Marvel's Agents of SHIELD until, eventually, Coulson discovered that TAHITI was actually the codename of a secret project that Nick Fury had developed to revive a fallen Avenger and had used it on Coulson after his death.
Marvel's Agents of SHIELD airs Tuesdays at 10 p.m. on ABC.