In a flashback where we see Marshall's many attempts of jumping across the buildings, we see him dressed as Dracula during the Halloween segment. Jason Segel is a huge fan of Dracula and once tried to write a Dracula musical involving just puppets. The idea was incorporated into his movie, Forgetting Sarah Marshall (2008).
This episode had been shot earlier, out of sequence, because Alyson Hannigan was still pregnant (baby born in March that year) & Cobie Smulders is barely showing at this point (baby born in May that year & would have been very pregnant, if filmed in order). They knew Lily would be on maternity leave in episodes towards the end of the season, hence filming this season finale earlier.
One of the scenes with Marshall on the roof previously used in the episode Three Days of Snow when future Ted gave his list of five words every man eventually says.
One of Marshall's flashbacks takes place during Halloween; there, a woman can be seen dressed in a giant pumpkin costume, possibly a reference to first season episode, The Slutty Pumpkin.
At the time of filming, Alyson Hannigan (Lily) is pregnant in real life. The camera angles strategically hide her belly throughout the episode. In the final jumping scene, they DO show Lily's belly (while she is jumping in the air) but not her face, indicating that a stunt double was used for this shot. When she lands her back is to the viewer, and when Ted lands Robin stands in front of her to cover her up.