Star Trek: 10 Crazy Facts About Klingons
1. Those Mysterious Forehead Ridges...
Back in the good ol' days of The Original Series, Klingons were an adversary of Starfleet, thanks to one of the worst First Contact incidents in the history of the Federation. But on The Original Series, Klingons had no forehead ridges. By the time Star Trek: The Motion Picture rolled around, they had ridges all of a sudden.
The issue was somewhat addressed on Deep Space Nine when the crew, including Worf, travelled back in time to the era of TOS and encountered some old school Klingons. The forehead ridge situation was a shock to his friends, with Worf's only explanation being that Klingons don't talk about it with outsiders.
Enterprise tried to explain it, though. Klingons originally all had ridges but then a virus caused their foreheads to become smooth, as a well as causing a change in their personalities. Apparently, the next phase of the virus may have even given them superpowers. Over time, the ridges returned and smooth headed Klingons became a rarity.
It was an interesting attempt to retcon an incongruity created by advances in SFX and movie makeup.