When James Franco and Seth Rogen are discussing the ending of the fictional Pineapple Express (2008) sequel some 12 minutes in, they are actually giving out the ending of This Is the End (2013). Franco acknowledges this in the ending scene.
Initially Michael Cera wasn't really slapping Rihanna's butt, but rather making the motion and stopping right before touching her. As a result, the scene wasn't working on camera because it looked too fake. Eventually Cera asked Rihanna if he could really slap her butt, she said yes but on the condition that she be allowed to really smack him across the face each time. Cera agreed.
The day Seth Rogen filmed the first scene of the movie where he goes to pick up Jay Baruchel at the airport, is also the same day he filmed a special feature for the movie Paul (2011). Rogen never bothered to change his wardrobe and is wearing the same clothes in both movies
When Jay and Craig are running from the dog-like demon in the neighbor's house, Craig saves Jay by pushing him out of a glass door. As Craig is jumping into Jay he is securely wearing the backpack full of food but when they crash through to the outside the very next second he is not wearing it.
When the beginning scene cuts to the hand-held camera following Seth Rogen, the camera which was filming Seth and Jay walk away from their meeting place can be seen.
When Jay and Seth make it to Heaven and see Craig, Jay starts yelling, "Yay!" as both Jay and Seth run to Craig. Jay yells the whole time until they reach Craig, but when they reach him the audio for Jay is still going while his mouth is closed.
Danny McBride: What the fuck is wrong with you, Franco? You have iPad's all over the goddamn walls in your house, but you jack off like a fuckin' pilgrim!