I will admit I know 'nuthin about Star Trek.
But, as a young lad I remember watching Trek with my dad (He loved it) and the Klingons had smooth heads and when Star Trek the motion picture came out my father and I went to see it and the Klingons had “lumpy” heads…what gives?
They evolved.
Actually, make up artists and special effects evolved. They had a pretty low budget in 1966.
It is not discussed in public. 
Better answer: improvements in makeup effects.
Yea but they still have SOME kind of explanation Huh?..
Actually, no. There was a DS9 episode where they went back to the time of the original series, saw the original series Klingons, and asked Worf what had happened. All he said was that Klingons don’t discuss it. I can only assume that their warrior culture forbids them to own up to massive continuity errors.
There was some ref works from people working on Treks aboot Hybrids and what not, but canon is silent on the matter.
Oh…OK
I wanted to hit Worf at that moment. I thought maybe they’d come up with a canon reason, but nope. Heck, on DS9 when they brought back Klingons who were originally on the old show (guess they live a long time) they suddenly had the foreheads.
Maybe it’s a problem with our TVs? 
Nah, videotape. Back in the '60s videotape just didn’t have the resolution to capture the forehead ridges.
It’s been a long time since I was a fan, but I seem to remember a reference in one of the books to the fact that there were two races of Klingons. The human-er-looking ones were holding most of the power in the Kirk era, then there was an ugly conflict of some sort, and the bumpy-headed ones became the dominant race.
You know, like, if you watched 1950’s television, you might not realize that human beings came in assorted colors…
Well, it sounded plausible to me, anyway.
Okay, here is what Michael Okuda had to say in The Star Trek Encyclopedia under the heading “Klingons”
In another book I can’t find right now (still packed, I guess), Gene had wanted to “improve” the Klingons for the new series Star Trek II, and would include in a story somewhere down the line an explanation for the differences. Since the new series never came about, instead being reworked into ST:TMP, this idea of The Bird’s was lost and he never got around to it on ST:TNG because he was possibly getting a little obsessed with his “technology unchained” vision of a future utopia and clearer heads wanted some action in the stories.
And now, in Enterprise, the Klingons have ridges.
Also, remember that the books (with a couple of exceptions that Aes can provide) are not canon in Star Trek.
So, for all intents and purposes, it really was all about budgets and makeup techniques.
I still want an explanation.
[Kirk]I want ANSWERS![/Kirk]
I’m waiting…
I thought the “official” answer was that the smooth-browed Klingons were Klingons who went through plastic surgery to disguise their looks, and make it easier to infiltrate human society.
Nope. The official answer is no answer.
It is a bit of revisionism on the part of the Roddenberry camp to say he wanted the foreheads from the beginning, or it was his idea. On the SE DVD of “The Motion Picture,” one of the make-up guys or one of the producers said GR didn’t like the idea of changing the Klingons at all, but finally gave in.
Of course, it later became “his idea all along.”
Sir Rhosis