Quote Originally Posted by Cikomyr View Post
That theory was kinda canonically nixed by the DS9 Episode Blood Oath, where TOS Klingons appeared with bumpy foreheads
Except that the question got raised right back up again in "Trials and Tribble-ations", also DS9, where the DS9 cast ended up in "The Trouble with Tribbles" ToS episode- quoth O'Brien, upon seeing the Klingons in the DS K7 bar:

O'Brien: "Those are Klingons?"
Worf: "We... do not speak of it."

So... uhm... yeah.