Revealing mistake: During the escape from Cloud City, watch as C-3PO's head strikes a lamp on the hull of the Millennium Falcon. It bends out of shape.
Revealing mistake: When Luke quits Hoth, the planet is actually painted on a glass sheet but the problem is that you can see a line joining two sheets. (00:36:15)
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)
Revealing mistake: When the Millennium Falcon is being pursued by the Star Destroyers after leaving Hoth, for a few seconds you can see the light of the ship through one of the Star Destroyers as the Falcon flies in front of it in the second shot. (00:35:25)
Revealing mistake: When Luke runs into Vader again after being sucked out the window, as they are fighting in the control room, watch closely: Vader's lightsaber hits the wall a couple of times, but the explosions are not where the saber touches the wall. (01:45:20)
Revealing mistake: Look carefully at the end of the cavern where the Millenium Falcon enters the giant asteroid. It's a dark blue circle, because the light is dim. But there is a little white circle on the right and the two circles are much brighter on the left side. It's because it's an image of a planet with a moon.
Revealing mistake: When we see the Star Destroyer for the first time, there are three of them on screen. The one on the top left, can still be seen even after another Star Destroyer came into frame and blocks the view.
Revealing mistake: When the Millenium Falcon fakes hitting the Star Destroyer, half of the sky is gray and the other half is black.
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)
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)
Revealing mistake: After Luke escapes from the snow monster, he collapses, and there's a shot of Han riding. When the camera pans down, there are no tracks in the snow where the tauntaun just ran. (00:11:00)
Revealing mistake: When the stormtroopers are firing at Chewie and the droids and C3PO is yelling at R2D2 about the Millennium Falcon's hyper-drive, there is a shot of Chewie being shot at and the beam putting a hole in the wall. In one shot, it doesn't look much bigger than a pinpoint, but in the next shot, the team at Industrial Lights and Magic were kind enough to add a CGI burn mark, which contracts, almost disappears, and then warps as smoke passes in front of it, wrecking the illusion. (01:48:10)
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