Answer: Klingon Skull Combs
In the original Star Trek TV series, Klingons had a smooth forehead like humans ̵
The only problem was that not a single word was spoken to explain why a whole was spoken. Non-resident species had acquired a very distinctive facial feature. The practical reason was that the movie had a much bigger budget than the TV series, and with a radically expanded special effects budget, the make-up artists and designers were wild. However, a universal explanation for the change was missing. In fact, so obviously absent-minded that the difference between combed and unkinned Klingons over the next few decades has become a constant debate and source of widespread fan-theories.
It was not until two episodes from the Star Trek: Enterprise aired in 2005, "Affliction" and "Divergence", that the difference was fully explained. In the 22nd century, Klingons experimented with genetic engineering of their own kind using DNA from genetically engineered humans. The experiments went awry and a virus strain caused a number of mutations, including a smooth forehead. Outside of the mutations caused by the experiments, all Klingons should have skull bones.