The franchise's fondness for helicopters (getting blown up) continues.
The one water control valve in the sewers of the Castle is a continuity nod to the many valve handles in Resident Evil's history.
On the elevator ride in the tower in Chapter 4-4, when Leon has to keep zealots from overloading the elevator in order to rise, one of the whiteheaded zealots in red robes is marked like a zombie. A continuity nod to when a Resident Evil game wouldn't be complete without a zombie.
The opening cutscene contains footage from cutscenes in the early games:
The missile used to destroy Raccoon City, which appeared at the end of Resident Evil 3.
The oncoming zombies, which use the zombie models from the old game engines, and look both crude and more than a little out of place.
In Separate Ways, one of Ada's reports references Luis sending an email for help to a college friend he was unaware had passed on. Ada intercepted it, which got her and Wesker involved. Apparently this friend was John, Ada's internal contact when spying on Umbrella in Resident Evil.
Leon tells Ashley "We're sandwiched all right" at the Cabin intro is call back to Barry's "You were almost a Jill sandwich" in the first Resident Evil game.
At the end of Separate Ways, Ada's helicopter flies off into the rising sun, just as the surviving members of the team do at the end of Resident Evil.
The stag's heads mounted on the wall in various places in the Castle seem a callback to the stag head in the aquarium room in the first Resident Evil.
Salazar becoming a giant plant thing that Leon must fight is a call back to Plant 42 in the original game. "Looks like we got to therootof the problem. - Chris.
Del Lago is a combination of the giant snake Yawn in Resident Evil and the giant crocodile in Resident Evil 2.
The dining hall with the paintings and clues is a call back to the previous painting puzzles in the first Resident Evil and Code: Veronica games. This really helps set the player up for the ambush.
The flying Novistadores around the hive are an improved remake of the bees and beehive in the first Resident Evil game.
The "dam blocking the waterfall to let the player access a secret entrance behind the waterfall" in Chapter 2-1 is the same in concept as what the player does to advance past in the barracks in the first Resident Evil game. The execution is vastly better. Perhaps a case of "this time we have the hardware to do this scene right."
Garradors are blind, dangerous, and hunt by sound. They can hear you if you run, but not if you walk. Just like lickers from Resident Evil 2.
Shooting the crates down to form a series of platforms across the water at the waterfall/dam reprises the crate pushing to form a platform across water in both Resident Evil and Resident Evil 2
The Cabin brawl in Chapter 2-2 with Luis is a much improved version of Leon travelling with Ada through the tunnels in Resident Evil 2. Ada will even kill all the zombies if Leon lets them get close enough.
The fight with U3 on the hanging rigs is a lot like the boss fight with William Birkin's dog form towards the end of Resident Evil 2. Birkin/It leaps up and down onto the tops of tall containers while Leon runs through the corridors between them.
The Separate Ways minigame, with the sequences and actions that are impossible to reconcile with the main game, is a shoutout to the same problems with the different character scenarios in the first two games.
The many rampaging trucks are a reference to the runaway truck in the opening cutscene of Resident Evil 2.
The gondola ride is a callback to the gondola ride in Resident Evil 2.
The Butler's letter Ashley finds isn't so much a call back as a near identical copy of the one in Code: Veronica.
Using an eyeball to unlock an entrance is a callback to Code: Veronica, where Claire uses an artificial eye in an anatomical mannequin to open the entrance to the cellar of the Prison Area.
Shooting out the first searchlight on the Military Base on the Island is a call back to Claire shooting out the searchlight when she first meets Steve in Code: Veronica.
Leon and Ashley counting to three then activating the switches simultaneously to unlock the door to the bulldozer/truck path is a reference to Chris and Claire counting to three and releasing the locks simultaneously to release the lock on the Linear Launcher in Code: Veronica.
Ashley uses a turnstile to open a hidden doorway to a secret area in Chapter 3-4, just as Claire does in the basement of the Prison area in Code: Veronica.
The flying robots in the boss battle with Krauser are a self reference to the little robots in Code: Veronica, where they summoned Hunters if they detected Chris.
The re-writable electronic door keys are a callback to the re-writable MO disks used to open the cell in the secret laboratory final level in Resident Evil.
Ada throws Leon a rocket launcher, from way up high, for the final boss fight. Just like Resident Evil 2.
Leon's shoulder sheathe for his knife is a callback to Chris and Claire's (non-functional) shoulder-mounted knives in Resident Evil 1 and 2.
Leon boosts Ashley up to get into the room with the Broken Butterfly, much as he did Ada in Resident Evil 2.
Saddler's "mouth eye" is a callback to William Birkin's first form in Resident Evil 2, which had an eye embedded in upper arm muscles. Neither is a suitable place for an eye, as it would be constantly deformed by changing pressures, leading to heavily distorted vision.
When Leon and Ashley do their slow motion leap out of the stained-glass windows in a cutscene in the church, it's a callback to Steve's slow-motion leap in through a window in a cutscene in Code: Veronica.
The typewriter save points are a major continuity nod with the early games, but their placement frequently makes no sense. This subverts one of the usual qualities of a save point, which is that they're usually placed at some kind of traveling nexus, after/before major fights, new equipment, etc. Resident Evil 4 has typewriters in place with no strategic places at all.
When Ada faces Krauser in Separate Ways, the cutscene opens with a view of a giant muscular arm wrenching loose a steel girder to use as a weapon. This is a callback to when Birkin's first form wrenched loose a metal pipe handrail to use as a weapon vs Leon in Resident Evil 2.
In the cutscene with Ada in Ch. 5-4 on the Island, Leon temporarily succumbs to the plagas in him. The camera cuts to a close-up of his face as his eyes burn red. There's an almost identical shot of William Birkin, right down to the burning red eyes, in Resident Evil 2 as he succumbs to the virus he had just injected into himself.
Verdugo, the inhumanly quick and tough enemy with blindingly fast melee attacks, is only weak to liquid nitrogen. And Verdugo's not all that weak even then. This is a callback to Resident Evil 3, where Nemesis is an inhumanly quick and tough enemy with blindingly fast melee attacks, and his main weakness is to freeze (liquid nitrogen) grenades. And it's not much of a weakness, merely a short slowdown, much as Verdugo is only slowed for a bit.
The optional crane in Chapter 5-1 (after the first Regenerators) is a callback to the short crane section in Claire's section in Code: Veronica.
When Krauser dies, he has soft tissue swell out of his chest and pulse, just like the weak spot in the first Tyrant battle in Resident Evil.
