Actor Allusion: Kiefer Sutherland was cast as the new voice of Snake in part as a reference to his role in as Jack Bauer in 24. After seeing Snake in the prison camp, Chico starts to panic and begins drawing attention to them. It doesn't take long for Snake to apply a sleeper hold on him while saying "Don't fight it."
Kiefer also faces off against beings that replicate humans, much like his fatherdid (he provides the vocals for Raiden in "Jamais Vu" mission).
Billing Displacement: Kiefer Sutherland's name comes last in the cast list of the opening credits of Ground Zeroes. But he is billed first in the end credits. This also happened to David Hayter at the beginning of Peace Walker.
The Cast Showoff: Not only did Stefanie Joosten provide the likeness, voice, and motion capture for Quiet, but she also sings "Quiet's Theme" on the soundtrack.
Many fans and sources speculated that The Phantom Pain was actually a Metal Gear game well before it was announced as such, thanks to the clues left in the original trailer.
A number of fans had figured out that the creepy child with a gas mask was actually a young Psycho Mantis, or that he was at least involved with the mysterious organization of "Those Who Don't Exist". This was given further credence by Game Informer about a month before the release of Ground Zeroes.
From his reveal, there were much speculation being thrown around about Eli being a young Liquid Snake, and with very little talk on the matter, only strengthened the suspicions. So, when it turns out that he actually is Liquid, no one was surprised.
Regarding a huge plot twist revealed at the end of the game: several people guessed that "Venom Snake" was a clone of Big Boss. While this isn't entirely true, he is a separate individual. Rather than being a complete clone, he made in to a body double with Magic Plastic Surgery and hypnotherapy.
Lying Creator: Hideo Kojima kept insisting that "Project Ogre" was not, in fact, a production title for Metal Gear Solid 5 prior to the reveal of The Phantom Pain. Turns out, it was. However, this is actually an instance of him being misquoted, because his original statement was in reference of Ground Zeroes at the time. Hideo Kojima also refersMetal Gear Solid V as a spin off and not a numbered entry. Which makes sense, considering you aren't actually playing as anyone from the Big Boss bloodline.
The first Ground Zeroes trailer was shown at a Tokyo event commemorating the 25th anniversary of the series.
Likewise, the trailers for the Déjà Vu and Jamais Vu missions were shown during the 15th and 25th anniversaries of Metal Gear Solid and Snatcher, respectively.
One of Us: Stefanie Joosten, who provides the likeness and voice of Quiet, is also a gamer and a longtime fan of the Metal Gear franchise. She even has a Twitch channel and a YouTube channel where fans can watch her play the game.
The Other Darrin: David Hayter was replaced by Kiefer Sutherland as the voice actor for Snake to help "recreate the Metal Gear series." Meanwhile, Ocelot is being voiced by Troy Baker instead of Patric Zimmerman. In the latter case, Ocelot at least has precedent, as the younger version of him was voiced by Josh Keaton in Metal Gear Solid 3 and Portable Ops. This also happened with Zero, as Jim Piddock thought the offer Konami made him was too little.
Paper-Thin Disguise: Meta example. The Moby Dick Studio marketing stunt was intentionally made as transparent as possible to get people hyped for The Phantom Pain.
Talking to Himself: Kiefer Sutherland voices both Snake and Ishmael, and the two share a scene together at the start of The Phantom Pain. It has been confirmed that he voices the medic that appears in Ground Zeroes, but his voice was altered digitally. The trope becomes literal when it's revealed that Ishmael is the real Big Boss, and Snake is really the medic who, through plastic surgery and hypnotherapy, has become Boss's body double.
Metal Gear Solid V was originally announced in two unrelated incarnations. The first was Metal Gear Solid: Ground Zeroes, a tech demo for Kojima Productions' new FOX Engine. The second, The Phantom Pain, was presented as a new IP from the mysterious Joakim Mogren's Moby Dick Studio. On March 27, 2013, series creator Hideo Kojima revealed that Ground Zeroes and The Phantom Pain were the prologue and main story, respectively, of a combined Metal Gear Solid V.
Former Big Boss voice actor David Hayter messed with the fanbase by leaving a lot of comments suggesting that he'd be involved in The Phantom Pain a small capacity and denying that he was involved at all.
After dataminers found evidence that there was a third chapter meant to be included in the game, Kojima made a series of tweets that featured the number 3 (such as complaining that a television show he likes never had a third season).
Urban Legend of Zelda: Fans of the Metal Gear series all over the world are beginning to believe that Chapter 3 is just hidden, not outright cut from the game. While Dataminers have mine the PC version and found no data of Chapter 3 aside from the title "Peace", the Console Versions have not been touched in any way. The console versions are 5GB larger than the PC version, so people are speculating that Kojima has been indeed playing the players "like a damn fiddle" and cut chapter 3 from PC but leaves the Console intact. The players are beginning to find out how to unlock it on Console. Whether or not this is true, the disappointed fans can now cross their fingers and hope for the best...
Several hallmarks of the other games were removed from the game in order to make the game more realistic and serious, such as Ration-based healing. Originally, Kojima thought about removing the trademark exclamation point, in favor of realistic expression from the enemy (as had been planned for MGS4 as well). Thankfully, the grandfather clause prevailed.
DLC was planned for Ground Zeroes beyond the Xbox/PlayStation exclusive content, but it was shelved so The Phantom Pain wouldn't be delayed.
For fans particularly disappointed by the game's ending and the manner of Eli and Mantis leaving,a reel detailing a "phantom episode" centered around them has surfaced from the CE bonus content. Eli would have absconded from Mother Base with the ST-84 Metal Gear, Mantis, and the child soldiers to form his "Kingdom of the Flies," and the episode would have culminated in a battle between the Diamond Dogs and Eli piloting the ST-84. After the battle, Eli would soon affirm to Snake his everlasting hatred, marking the genesis of his character as it would be known in Metal Gear Solid. The fact that so much of Eli's storyline and development seems to have been compressed into this episode suggests that this was to be an intended finale of the game. The reason why it was cut is currently unknown, but there are many different possible reasons, the most likely of which are either a), the content (after all, you are bombing a bunch of child soldiers) or b), the Konami-Kojima dispute mixed with budget problems. This case is rather unusual seeing as it was such a key portion of content. It is yet to be learned if the episode will be released as DLC.
In addition to the cut "Kingdom of the Flies" mission detailed above - and presumably the cut content to the second chapter - a title card under the name of Chapter 3 - "Peace" was found during datamining of the game's files, which means an entire third chapter may have been cut.
At one point you were supposed to be allowed to bring the Battle Gear into missions, but this was scrapped because it was too much of a Game Breaker.
Prior to release, it was stated that everyone would be able to create their own unique Mother Base. While not outright confirmed, this appears to have been scrapped due to time constraints, as everyone has the exact same Mother Base apart from color. To that end, the proposed feature allowing players to access their Mother Base using the smartphone app was also cut.
Another feature mentioned prior to release was that players importing data from Ground Zeroes could return to Camp Omega. However, it seems that this was removed.
There is a groove on the back of Sahalenthropus that suggests that it would have been combined with an AI Pod similar to Peace Walker. Handwaved in-universe by Skull Face, who adamantly refused to allow Sahalenthropus to be controlled by AI, referencing what happened to Peace Walker in the process.
Data miners have found audio files in the game indicating that Skull Face was going to be a boss battle. And following the completion of the battle the player would have been given a choice: either kill him or take him back to Mother Base as a prisoner.
Even though the helicopter that served as your support in Ground Zeroes, here referred to as an HP-48 Krokodil makes multiple appearances as an enemy helicopter and would serve as an excellent support helicopter, no option to swap to it exists, and the default UTH-66 Blackfoot helicopter is the only option available.
Word of God: According to the Piggyback official strategy guide for The Phantom Pain, the enemy commander who takes control of one of Mother Base's platforms in Mission 22 ("Retake the Platform") is actually the same soldier who helped Miller to his feet during the XOF attack at the end of Ground Zeroes.
Word of Saint Paul: Quiet's voice actress, Stefanie Joosten, personally disproved the "Chico is Quiet" theory. This is confirmed in-game when it is revealed that Quiet is dressed the way she is because of her getting set on fire by Ishmael and Ahab in the prologue.