Ok, I'd like to avoid all the "When Baraka gets knocked into the fire in MKA, it's rain!" stuff in this thread, and actually limit it to info people really might not know.
Keeping that in mind, I'd like people to add their own stuff, just not the completely obvious.

So this is what I've noticed / found out over the years:

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MK1 -
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- Budget of $20M US
- (Thanks to Ghostdragon for the gross totals)
-Mortal Kombat-
Domestic Gross: $70,454,098
Foreign: $51,741,822
Worldwide: $122,195,920


- Along with Sub Zero, Scorpion leads out the warriors in the “And now for a taste of things to come” scene. He then disappears... (Cut fight scene perhaps? It's rumoured to be a scene where he does a fatality)



- The island’s mountain in the 1st movie represents the MK2 Battle Plan



- The promotional poster seems to remind you of the MK1/2 battle plan as well, specifically Shao Kahn atop the MK2 mountain (but for the movie, Goro, obviously):



- Liu Kang knocks Shang Tsung into the “Pit” (Ok, that one's pretty obvious)



- The arena where Liu Kang fights Reptile has the same window / design as MK2’s Tower stage (thanks to TMK for this one):



- The arena at the start of the fight between Cage and Scorpion is reminiscent of MKII's 'Living Forest"



- Jax is listed as Jaxx in the credits



- Liu Kang is dressed in his MK1outfit for the final fight vs. Shang Tsung



- Trevor Goddard (Kano) died in 2003 from a drug overdose.

- Gregory McKinney (Jaxx) died of a brain anneuryism in 1998 (according to imdb)

- Shang Tsung uses the phrases "Fatality, Flawless Victory and Finish him"

- At the end of the credits, you can hear 3 of Shao Kahn's phrases from MKII: "Excellent, Hahahahahaha, Flawless Victory"

Shao Kahn utters two MK2 game phrases as he appears at the end of the movie: "Bow to me," and "You weak, pathetic fools..." (changed slightly from 'You weak, pathetic fool' - his in-game taunt.)

- On the boat, when Shang Tsung announces "It has begun" - you see an image of Shao Kahn's helmet. (which strangely doesn't look like Shao Kahn at the end of the film...



- Shang Tsung’s boat was originally called the Dragon Wing in the novel

- Liu Kang never once strikes or attempts to attack Kitana in their fight scene, prefering to parry and block her attacks.

- Near the end, when Shang Tsung says 'Seize them" - Kitana exerts her authority (as Princess of Outworld) over the Shadow Priests and commands them to; "Stay where you are!"

Speaking of that scene, here you can see where they got the inspiration for Shang Tsung's Shadow Priests:



- The Outworld is based on the Wastelands of MKII:



- Scorpion uses the game phrases; "Get over here" and "Come here" when talking to his spear. He also uses a made up one that I like, "Get down here", when fighting Johnny Cage.

- During the end credits, an MK3 kombat kode is presented intermittently (I've added the MKIII atrraction message as well):



Haven't had chance to find out which kk that is yet, but I will update when I do.

- Also in the end credits, there is an advertising message for "Otomix clothing".



Otomix were originally planning to advertise in MKII, on Johnny Cage's pants. However, they pulled out at the last minute - probably to avoid a boycott of their product due to the backlash of negative publicity surrounding MKII's blood and gore. The 'otomix pants' can still be seen if you perform Shang Tsung's soul steal fatality on Johnny Cage in MKII:



Or here

- There are a few rumours abound about a couple of deleted scenes from the first movie. Now, I'm not sure if they are from an early version of the script (that somehow someone saw) or just from the novel related to the movie, still -they do sound rather plausible. Here is the info (from imdb):

Scripted but not filmed was a scene where Shang Tsung allowed the "Kombatants" a night to bury Art Lean and mourn his loss. They buried him the Garden of Statues, underneath the statue of Kung Lao; this is the only place where Kung Lao appears in any of the movies. Also scripted but not filmed was a battle between Sonya Blade (who wins, naturally) and Jade, another of Shang Tsung's bodyguards.


(Thanks to superliu for this one) Liu Kang may perform the cartwheel part of his MK1 & 2 Fatality in the first fight of the tournament. It is defintely very similar, though he uses the stick for balance, and could very well have been inspired by the game.



-----------------------------------
MK1/MK2 special moves:
-----------------------------------
- Johnny Cage performs a Shadow Kick (vs. Scorpion) and a Splits Punch (vs. Goro)
He also performs (accidently) his MK2 Friendship: (thanks to RulerofFire-Inferno for reminding me of this one)



- Scorpion performs a Spear (against Cage), a teleport punch (w/out the punch against Cage) and attempts a Toasty fatality

- Kang performs a Bike Kick (vs. Reptile) and a fireball (vs. Shang Tsung)

- Sonya performs a Leg Grab (vs. Kano) and Neck Snap fatality(?)

- Sub Zero performs an ice blast / ice shatter fatality (vs. fighting monk)

- Reptile uses his acid spit (vs. Liu Kang) and invisibility

- Shang Tsung morphs (into Master Boyd and Chan Kang) and uses his Soul Steal fatality

- Jax, Kitana and Shao Kahn never used any of their special moves from the games

- Sonya uses both an uppercut (disarms Kano of his knife) and sweep (vs. monk in the dining room)

- Kano performs a knife swipe (vs. Sonya)

===

MKA -

===


-Mortal Kombat: Annihilation-
Domestic: $35,927,406
Foreign: $15,449,455
Worldwide: $51,376,861
- Raiden defeats the final Raptor by uppercutting him through the roof, then follows him up before seeing he is defeated. This is reference to the two tiered levels in the MK3 series of games.

- MKA had a budget of $30M US

- The elder god of wind is the one to lie to Raiden in the Temple of the Elder Gods. At the end, the fire and water gods reprimand Shinnok. You can assume then, that Shinnok was the elder of wind (which kinda connects to the father / son plotline)

- Mileena, Baraka, Cyrax, Noob Saibot and Ermac appear in Mortal Kombat: Annihilation, but are never mentioned by their MK game name. All but Cyrax are also left out of the movie theme song.

- Kung Lao, Kintaro, Sektor, Kabal, Kurtis Stryker are the only characters from the first 3 Mortal Kombat games that never appeared in either MK movie.

- Liu Kang performs his MKII/MK3 dragon Animality (if you can call it that)

- Like in the first film with Kitana, Liu Kang never once strikes or attempts to attack Jade in their fight scene, prefering to parry and block her attacks.

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MKTJB -

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- The floor of the arena where the Great Kung Lao fights Goro has the same face as the floor of the Goro vs. Art Lean fight in the 1st MK movie.

- Scorpion and Sub Zero, who are inaccurately dubbed as being from Outworld, seem to have their original fight for the 'honour of their clans' in MKII's Wasteland.

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MK Conquest

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In the first episode of MKC, after Kung Lao defeats Shang Tsung he performs a MERCY, allowing him to live rather than kill him like Shao Kahn wants him too. Obvious reference to the MK3 series of games.

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I'll add more when I remember them, feel free to add your own interesting info. MKC stuff is welcome too.