Continuity mistake: When Luke is carrying Yoda on his back, as he says, "But tell me why I can't," his head is turned to his left, but in the previous and following shot, he is facing forward. (00:39:55)
Other mistake: When in the cave, Leia falls and is caught by Han. She tells him that "being held by you isn't quite enough to get me excited." As she is saying this you can see Harrison Ford mouthing her exact words. (00:44:05)
Continuity mistake: When Han and Leia kiss in the Millennium Falcon, Leia's arms are around Han's head in the wideshot, but not in the close-up. (00:49:55)
Factual error: There are three major mistakes regarding gravity in the escape sequence in the asteroid field. Han and Chewie take the Millennium Falcon to refuge on an asteroid that, while visually huge, is still far too tiny to have sufficient gravity to allow humans to walk in anything like a normal fashion; yet, they walk normally both inside and outside of the ship. They also do not use pressurized suits outside of the ship, even though the asteroid's gravity should be far too weak to accumulate any significant atmospheric pressure; they use oxygen masks, but their blood should have boiled in near-zero atmospheric pressure. Finally, and most ridiculously, they fly straight down the giant cave worm's throat and land on the side of its throat (this is obvious in the shot where the Millennium Falcon lifts off and heads toward the toothy exit), and they get out and walk around on the side of its throat, which would mean the asteroid's gravity was impossibly perpendicular to its mass. (00:57:50)
Visible crew/equipment: This can only be seen in the widescreen version. When Han, Leia and Chewie venture out of the Falcon to investigate the cave they're hiding in on the asteroid, as they walk towards the left of the screen you can see the edge of the tarpaulin sheet they're standing on as well as the faint outline of a crew member trying to get out of the camera shot. (00:58:40)
Continuity mistake: In one shot while Luke is being trained on Dagobah, look at R2 when Yoda says, "Anger, fear, aggression, the dark side of the force are they." The little projection nozzle that sticks out of R2's head is on the left side of the red light (it's on the right side for the rest of the movie). (01:01:25)
Continuity mistake: When Vader addresses the bounty hunters, Piett says, "Bounty hunters-We don't need their scum.", and starts walking towards a corner. In the next shot, he is already at that corner. (01:03:20)
Continuity mistake: When Luke uses the Force to lift a rock, the rock is much higher above the ground in the close-up than in the wide-shot. (01:05:45)
Continuity mistake: When Luke uses the Force to lift a rock, there are different rocks in the wideshot and the close-up, and the distance between the rocks on the ground changes between the two shots. (01:05:45)
Continuity mistake: When Luke is standing on his hands while using the Force to lift a rock, he is distracted. As he collapses, the rocks on the ground are different and in a different position from the rocks seen in the shot before Yoda says, "concentrate." (01:06:00)
Revealing mistake: When Yoda is training Luke by having him do a handstand and stack some rocks, watch closely during the close-up of one of the rocks. You can see a string holding it in the air. (01:08:30)
Continuity mistake: When Yoda levitates Luke's X-wing, Luke comes over to watch, and his jacket is open. Two shots later, his jacket is closed. (01:09:05)
Revealing mistake: When Captain Needa is killed by Darth Vader because they lost track of the Millennium Falcon, in the background, two guards come to take him away. You can clearly see the dead captain get up almost by himself. (01:10:50)
Continuity mistake: Widescreen version: After Vader chokes Captain Needa, he talks to Admiral Piett. The distance between Piett and the officer next to him differs between shots, depending on the camera angle. (01:10:55)
Continuity mistake: After Luke says to Yoda, "You want the impossible," Yoda's position jumps from the ground to a rocky boulder, where he then lifts Luke's X-Wing fighter from the swamp. (01:11:30 - 01:13:05)
Continuity mistake: When Luke is doing a handstand and has the vision of Han and Leia, in the last close-up of him upside-down, his elbows are bent, but in the next shot, his arms are fully extended. (01:14:05)
Revealing mistake: After Princess Leia switches C-3PO off and the camera angle switches to a close up of Han and Leia discussing where to go next, you can see C-3PO moving in the background as the actor inside the suit is breathing. (01:16:10)
Continuity mistake: When the Millennium Falcon is approaching Cloud City, Leia's hand is on the top of Han's pilot seat. In some shots, her hand is in the middle of the seat, in other shots, it is right next to the middle. (01:18:35)
Continuity mistake: Widescreen version: When Leia, Han, Chewbacca and C-3PO are first walking through Cloud City, as Han says to Lando, "You sound like a businessman", Chewbacca is walking behind Leia. In the next shot, he is ahead of her, and then seems to walk to the side to wait for her. In the following shot, he is behind her again. (01:18:45)
Revealing mistake: After C-3PO gets shot, as Chewbacca walks over to the door, his right foot crushes one of the steps on a small set of stairs. (01:19:25)
Answer: The short, short answer to this is "Yes... from a certain point of view." The long answer is complicated and depends completely on what timeframe you mean by "always." If you're going back all the way to the early rough drafts of the early-mid 70s (which actually resemble Episode I more than they do the Star Wars of 1977), you'll find there's a cyborg father figure protagonist that makes a heroic sacrifice, and then another character that is a "black knight" villain that eventually turns to the side of good near the end. Just to make things more complicated, there is yet another character, a villain by the name of "Darth Vader" that is a human Imperial officer like Grand Moff Tarkin. It may be a stretch to count all that as "Darth Vader was always the father" but the pieces were all there, at least.
TonyPH
(1) Now the earliest explicit mention on any documented material that Darth Vader is Luke's father comes from notes Lucas made outlining the general story of the trilogy and its place in the larger Star Wars saga. These were found in the archives for The Empire Strikes Back, but they are undated and we don't know if they were written before Star Wars (1977) and carried forward, or if they were written afterward. These were found fairly recently (made public in 2010) and as far as I know Lucas has never commented publicly about them.
TonyPH
(3) One thing we know, at least, is that Lucas had come up with the idea of Darth Vader the father before starting work on The Empire Strikes Back. Something incredibly odd, though, is that the first draft written by Leigh Brackett does not feature the twist (and in fact introduces Anakin himself as a ghost); for a long time many fans took this as proof that Lucas hadn't thought of the idea at all by then, but after the series outline was discovered it was made apparent that Lucas simply hadn't told Brackett for some reason. Perhaps he wasn't sure yet that he wanted to go through with it, or maybe at that point he was thinking of revealing it in the third film. Either way, Lucas would write the second draft himself, and that's where the twist first appears in script form.
TonyPH
(2) Something that must be understood about Star Wars (1977) is that it was an ALTERNATIVE to his original plans of a saga. By then he didn't think it was realistic that he would be able to make a long series of many movies, so he came up with a "Plan B": he crammed the general story of the trilogy into one movie. So we know that when Star Wars (1977) was filming, Darth Vader was NOT Luke's father, because this one movie was IT, that was the whole story. But what we DON'T know, is whether that means Lucas had abandoned the idea of Vader being the father in order to simplify the story, or if Lucas simply hadn't thought of that at all just yet.
TonyPH
(2, cont.) On a side note, you can tell by watching Star Wars (1977) how it has condensed the story of the trilogy. The middle portion has the characters trying to escape capture from the Empire while one of them loses a duel with Darth Vader (like The Empire Strikes Back) and the third act is a final battle against the Death Star above a forest moon (like Return of the Jedi). The first act features a member of royalty on the run while a couple of protagonists find the main hero on a desert planet, resembling the original drafts and by extension Star Wars: Episode I. Because of this we've arguably never actually had a "pure" first chapter to the original trilogy, even though Lucas eventually had the film serve this purpose anyway.
TonyPH