Audio problem: After Han goes to the command centre during the battle on Hoth, Leia says to a controller, "Send all troops in sector twelve to the south slope to protect the fighters," but the words don't match her lips. (00:30:35)
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: When a blast rocks the command centre during the battle on Hoth, C-3PO is thrown backwards into Han's arms. As the wideshot leaves, Threepio is leaning backwards, supported by Han, but in the next shot, both of them standing upright in a normal way. Toryn Farr, the woman sitting behind them, has changed position as well. (00:30:40)
Continuity mistake: When Chewbacca is choking Lando, as Leia says, "We don't need any of your help.", Lando is standing on his knees. In the next shot, only his head can be seen, but the position of his head and Chewie's arms shows that he is now standing straight up. In the next shot, he is on his knees again. (01:38:15)
Continuity mistake: After the carbonite block is pulled out of the carbonite freezer, there's a close-up of Leia leaning towards Chewbacca. In the next shot, she is not leaning on him, and then Chewie pulls her towards him again. (01:33:05)
Revealing mistake: In the first shot of Luke riding on Hoth, the tauntaun leaves no tracks in the snow. (00:02:50)
Audio problem: In the carbon freezing chamber, Han says to Chewbacca, "The Princess - you have to take care of her". When he says, "You have to take care of her", his head is shown from behind, but the motion of his jaw doesn't match the words. (01:35:35)
Continuity mistake: When Luke is holding on to the antennae-like assembly underneath Cloud City, right before and after the hatch closes, the distance between a thin pole and his left arm keeps changing between shots. (01:48:25)
Continuity mistake: When Luke is first attacked by the wampa (ice creature), the amount of blood on him changes between shots. (00:04:00)
Continuity mistake: When Luke summons his lightsaber back to his hand during the duel, he's holding onto a hose with his other hand. He is holding it differently from the previous shot. (01:40:25)
Continuity mistake: When Luke is attacked by the ice creature all the scars are on the right side of his face, but right before he talks to Obi-Wan you see a big scar on the left side of his face, then when he talks to Obi-Wan you can see there are no scars on the left side of his face. (00:11:00 - 00:13:05)
Continuity mistake: When Lando releases Leia and Chewbacca, as Leia says, "What do you think you're doing?" her body is turned in a different direction from the previous shot, and she's holding the blasters differently. (01:37:55)
Continuity mistake: When Janson (Wedge's gunner) fires the harpoon at a leg of an AT-AT, in the first shot of the snowspeeder flying around the AT-AT, the speeder casts a shadow on the snow. In all other shots of this sequence, there is no shadow of the snowspeeder.
Revealing mistake: Right after Luke and Darth fight for the first time, Darth gets kicked off the Carbonite freezer. Then Luke jumps down to fight him again. After he jumps, you hear what sounds suspiciously like a trampoline. Then in the lower right screen, you can see Luke's head pop back up into frame. Widescreen Version only, although the sound is audible on the fullscreen version too.
Continuity mistake: When Han is trying to keep Luke warm, his rank insignia badge alternates between being on the right and left side of his chest. The amount of snow on Han and Luke's clothes also changes repeatedly, especially Luke's. (00:14:15)
Continuity mistake: During the escape from Hoth, C-3PO is far behind Han and Leia as they are running to the Millennium Falcon after the cave-in blocks their path. When they reach the door, he is much closer to them. (00:32:40)
Revealing mistake: When the medical droid tests Luke's mechanical hand at the end of the film, his thumb moves before the "muscle." (02:00:40)
Continuity mistake: When Luke leaves Dagobah, he uses a ladder to climb into his cockpit, but when he sits in, it has disappeared.
Continuity mistake: When the Millennium Falcon jumps into hyperspace at the end of the film, Leia, Luke and Lando are in the cockpit, being tossed back. In the next shot of the cockpit, Lando is missing. (01:54:20)
Revealing mistake: In the original (not Special Edition): When the AT-ATs are attacking the Hoth base, they do a camera angle from inside Luke's cockpit. As his snowspeeder is passing through the legs of an AT-AT, part of the cockpit becomes translucent and you can slightly see the AT-AT through a metal bar.
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