10 Changes Star Trek Hoped You Wouldn't Notice
5. The Trill
The Trill were introduced in The Next Generation, with Ambassador Odan being their first representative in the franchise. He failed to mention to Dr Crusher that the Trill were a joined species, one who was unable to use transporters for that reason. They are bore distinct forehead ridges in this appearance as well, shown on both the male and female hosts for the Odan Symbiont.
Then, along came Terry Farrell.
When Deep Space Nine was entering production, she was tested with this make-up and for several reasons, it was quickly abandoned. First, she has skin that is so sensitive that she can barely stay in direct sunlight for more than a few minutes, so heavy prosthetics were out of the question. Second, hiding her face behind these ridges was sacrilege of the highest order!
The Trill were then redesigned with the spots that ran from the forehead to toes, revealing far more of the actors' faces going forward. Naturally, this change was never addressed on-screen by the writers and the characters, so it is up to the audience to decide what exactly prompted this enormous change in Trill physiology in those few years.