I refer of course to the Klingon Forehead. What did happen between the 22nd and 24th centuries?
From StarTrek.com :
The Klingon forehead issue is a complicated one, because it has to take into account a number of factors if one is to stay true to the Star Trek universe as previously established (related feature). The simplest theories (e.g. "southern" vs. "northern" breeds) have to be thrown out because Star Trek: Deep Space Nine reprised three Klingon characters from the Original Series — Kor, Koloth and Kang — and gave them forehead ridges in "Blood Oath." Then there was "Trials and Tribble-ations," where the crew went back in time to Station K-7, looked around and said, "Those are Klingons??" They turned to bumpy-headed Worf for an explanation, and he would only say it's a long story and "We do not discuss it with outsiders."Well said.
For the mainstream hypotheses advanced to explain this conundrum, the Jannissary, Evolutionary, Bio-Agent, Surgical, Hybrid, Cosmetic, Q-Continuum, and "Under-Resourced Creator" hypotheses, just read the StarTrek.com article on Klingon Discrepancy theories.
These make far more sense than some of the arguments advanced in favour of "Intelligent Design", the "Elders of Zion", or that the Rathergate memos are genuine.
Hat Tip : Alan K. Henderson over at Sasha Castel