Continuity mistake: After Luke is sucked out the window, he grabs hold of two bars in a railing, and his right hand is above the left one. As he hoists himself up in the next shot, it is the other way round: his left hand is above the right hand.
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 R2-D2 opens the door to the landing platform during the escape from Cloud City, the droid exits through the door. Some shots later, he is back inside. (01:43:55)
Continuity mistake: There is a shot from inside the cockpit, just before Luke's snow speeder crashes. Look at the speeder's legs, the proportions are wrong. According to this shot the snow speeder must be tiny.
Continuity mistake: On Hoth, the hangar's door starts closing twice. (00:11:45)
Continuity mistake: During the duel in Cloud City, Darth Vader cuts three pillars with his light saber, just before cutting Luke's hand. The pillars aren't burning after that yet in the following shot, they are burned. (01:45:55)
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)
Continuity mistake: Leia and Han become annoyed with C3PO's chattering, and Leia turns him off. However, if you watch him in the background, you can see his body move as he breathes.
Continuity mistake: During the lightsaber duel, Vader hooks Luke's lightsaber out of his hand. When Luke summons it back to his hand, it is not lying in the same place as it landed. This can be seen from the lines on the floor.
Continuity mistake: Widescreen version: Darth Vader and Admiral Piett are talking on board the Star Destroyer at the end of the movie. In the first shot, a man is standing behind Piett. In the next shot of Vader and Piett, a different man is standing behind Piett. Then, in the third shot of Vader and Piett, the first man is back. The design of the wall and window also differs. (01:57:05)
Continuity mistake: When Vader is about to tell Luke that he is his father, as Luke says, "He told me enough," he is holding onto one of the thin poles next to him. In the next shot, his hand is positioned lower down and clutching the other pole. Also, his head is in a different position, seen from the markings on one of the other poles. The position of his head and hand changes in the next shot as well. (01:46:35)
Continuity mistake: When Luke and Vader are fighting in the carbon-freezing-chamber, Vader says, " You have learned much, young one." Their light sabres are held in different angles between this shot and the previous and following shot. (01:39:40)
Continuity mistake: When the officers on Hoth are discussing that the shield must be shut, Leia is standing in front of a spaceship. Immediately, she is several feet back leaning against the spaceship.
Continuity mistake: Widescreen version: When Luke and Vader are fighting in the carbon-freezing chamber, as Vader says, "Only your hatred can destroy me.", the hose hanging from the ceiling behind Luke is pointed straight down, but in the previous shot, it was bent. This has nothing to do with camera angles - watch the smoke coming from it, it changes direction as well. (01:40:40)
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 first meets Vader in Cloud City, in the shot from behind him after he walks up the stairs, his arms are positioned differently from the previous shot. (01:36:55)
Continuity mistake: When Luke is hanging upside down in the monster's cave he uses the Force to get his lightsaber. In the shot from Luke's POV it's about 3/4 buried in snow, but in the reverse it's only about 1/4 buried.
Revealing mistake: When the Millenium Falcon fakes hitting the Star Destroyer, half of the sky is gray and the other half is black.
Other mistake: When the Falcon speeds away from Hoth just as Darth Vader steps out, watch carefully as the Falcon exits the hangar. You will notice that a very small spot of the Falcon is visible through the roof of the hangar as the Falcon exits and flies upwards.
Revealing mistake: Before Lando warns the people of Cloud City that the Empire has taken control of the city, Leia shoots a stormtrooper. The burn mark can be seen on the troopers armour before the laser beam hits him. (01:42:40)
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