Ah, retcons. One of my favorite games.
Star Trek
Seeming plot hole/inconsistency:
Klingons looked very different in ST:TOS from their appearance in later productions in the franchise. Warf would only say that they (Klingons) do not discuss the matter.
Perfectly logical explanation:
At some point in Klingon evolution, two major populations became geographically isolated. Inevitably, divergence follows isolation, and two distinct phenotypes emerged. For convenience, we’ll call them strains “B” and “S” (for “Bumpy” and “Smooth”).
Strain B’s environment was dominated by harsh, open terrain unsuitable for agriculture; they maintained a nomadic hunter lifestyle. This encouraged the conservation of traits valuable to predators–aggressive tendencies, increased size and strength, bony armor over vulnerable areas, and so forth. The early culture of the Bs was dominated by the hunter/warrior, and over time this became institutionalized by social structures organized around aggression and personal honor and prestige.
Strain S developed in a more constrained environment, favoring forests over plains. It is even possible that this strain had an arboreal stage, in which lighter individuals would be at an advantage. Great size was counterproductive beyond a certain point, as it interfered with movement. More abundant plant life led to greater emphasis of food-gathering over hunting, and to earlier development of agriculture. This, in turn, led to more cooperative structures as the foundation of S culture.
Eventually, of course, the two groups encountered one another. This led to immediate aggression by the Bs and frantic defense by their smaller cousins. Eventually, the S strain was subjugated; they became a peasant caste of sorts, laboring for the Bs. The necessities of survival among their brutal, domineering kin began to reshape S culture into a thing of diplomacy, subtlety, and treachery. Such efforts slowly eased the oppression of the S strain; combined with the demands of an Industrial Revolution, it led to more prominent roles for members of strain S. S Scientists played a prominent role in the dawn of the Klingon Space Age, and for various reasons (including low body mass and calmer temperament), the first Klingon astronauts were of strain S.
This set a pattern. Early scientific and exploratory vessels were crewed almost exclusively by S individuals, both because they required less resources and because they were less likely to kill each other on long missions. Occasionally, the Bs demanded command of such expeditions, but such efforts were sporadic, and eventually ceased entirely. Only when space travel advanced to a point at which a battleship was possible did the Bs take interest again. However, partly due to treachery by S agents, efforts by the Bs to command and crew ships frequently met with disaster. The incidents with Klingons in Star Trek: Enterprise occurred during this period–indeed, the Klingon passenger the Enterprise returned home may have been an indirect victim of such treachery. In time, the Bs abandoned space flight, except for times of war, colonization efforts, and occasional travel.
Thus, the Klingons encountered during the period of TOS were exclusively of strain S, and they dealt with other races with the same underhanded and treacherous tactics they had long practiced against their own kind. Expanding territory and the occasional exhilirating war kept the Bs happy for some time. A series of incidents–not least of which were the creation of the Organian Peace Treaty and the matter of some poisoned grain at Station K-7–caused some Bs to begin taking notice of the S’s actions. At some point during the period between the end of TOS and the events of The Search for Spock, the S’s double-dealing ways became common knowledge. The Bs, with their honor-based culture, were shamed and outraged, and civil war soon broke out. The S strain was virtually wiped out; perhaps some secret colonies remain, but otherwise, their only legacy is in the descendants of mixed blood that had been growing increasingly common. The Bs assumed more direct control over their Empire, and the rest is history.
And there you have my B-S theory on the difference between the TOS Klingons and the others. 