The crew of Supernatural loves to echo past events using imagery and dialogue, sometimes obvious, sometimes subtle. They're mile markers along the journey that encourage the audience to look back and see how far the show has come.
Here's a few that I've noticed;

Pilot: The obvious mirror image of Mary and Jess both pinned to the ceiling.

Pilot: John hands baby Sammy to Dean and says "Take your brother outside as fast as you can. Don't look back."

Home: Sam says to Sari "Take your brother outside as fast as you can and don't look back."
[both events happened in the same house]




Pilot: Sam throws a gun into the trunk of the Impala and says "We got work to do."

All Hell Breaks Loose Part 2: Dean throws the Colt into the trunk of the Impala and says "We got work to do."
[both scenes shot from inside the trunk]

Salvation: Sam and Dean sneak into a house and are confronted by a man, Dean restrains him and tries to convince him that they are there to help. The man yells to his wife to get the baby and get out. Dean knocks him unconscious, slings him over his shoulder and carries him to safety as Sam goes upstairs to rescue the man's wife and baby.
No Rest for the Wicked: Sam, Dean and Ruby enter a house and are confronted by a man. Dean restrains him and tries to convince him that they are there to help and he should hide in the basement. When the man protests that he won't leave his wife, Dean knocks him unconscious, slings him over his shoulder and carries him to safety while Sam and Ruby go upstairs after the mother and child. Only this time Sam's there to kill the child, not save her.


Devil's Trap: John tries to get Dean to give him the Colt on the pretense that he'll use it on the the yellow-eyed demon. Dean recognizes that John is possessed and YED reveals himself, disarming Dean and restraining both the boys. YED tortures Dean as Sam is forced to watch.
No Rest for the Wicked: Ruby tries to get Sam to give her the knife on the pretense that she'll use it to fight off the hellhound. Dean recognizes that she's not Ruby and Lilith is revealed. She disarms Sam and restrains both of the boys. Dean is ripped to shreds while Sam is forced to watch.




Born Under A Bad Sign: Sam picks up Dean's gun and offers it to Dean. "I don't wanna hurt anyone else. I don't wanna hurt you."
Heart: Madison picks up Dean's gun and offers it to Sam. "I don't wanna hurt anyone else. I don't wanna hurt you."

Folsom Prison Blues: They get themselves arrested so they can hunt a ghost and Sam complains about how crazy their plan is. He questions why they're putting their neck on the line for the guy who asked for their help. Dean replies that the guy saved Dad's life and they owe him.
Sam, Interrupted: They get themselves admitted to a mental institute so they can hunt a ghost and Dean complains that he can't believe Sam talked him into this. Sam replies that the guy who asked them for help saved Dad's life and it's the least they can do
[interesting sidenote: Dean seems more comfortable surrounded by criminals than he does by crazy people while Sam's the opposite].

Pilot: Sam is sleeping in bed with Jessica when he is awakened by a noise. He goes to investigate and attacks a dark figure that turns out to be Dean. They are silhouetted against a window as Sam asks what he's doing here. Dean replies that he's looking for a beer.
What Is And What Should Never Be: Sam is sleeping in bed with Jessica when he is awakened by a noise. He goes to investigate and is attacked by a dark figure that turns out to be Dean. They're silhouetted against a window as Sam asks what he's doing here. Dean replies that he's looking for a beer.

Pilot: Dean goes to Sam, asking for help in finding their missing father. He says "I can't do this alone." Sam says "Yes you can."

99 Problems: Dean tells Sam he's thinking of giving up and Sam says he can't, that he needs his help. "I can't do this alone." Dean walks out.



Well, this article isn't really finished but I've decided to publish it anyway. It's been kicking around for long enough and Kristine's pick about the line in 99 Problems reminded me of it.

So I figure you guys can help me finish it by posting the things that you've noticed and I'll edit it with your contributions!