10 Times Star Trek Broke Its Own Rules
3. The Trill Species
The Trill species were introduced in Star Trek The Next Generation in the form of Ambassador Odan, a new love interest for Dr. Crusher. He neglects to tell her that the Trill are a joined species and, when the host is killed in a shuttle, the symbiont is transferred to Riker. Later, the symbiont is again transferred, but this time to a female host - something that causes issues of its own.
There are a number of issues here that are quietly ignored by the Trill of Deep Space Nine. First, Odan was on the shuttle as he was worried about the effects of the transporter on the symbiont, claiming that transportation could damage it.
Second, Crusher is able to move the symbiont from Riker and then to the next host with little to no effort, a feat that would be marvelled in Deep Space Nine. Ezri Dax further makes a point of this by saying she would have preferred only to be a temporary host, but once a host and symbiont are joined, that is the end of it.
Lastly, the Trill get a complete appearance overhaul. This is due to Terry Farrell - she is such an attractive person that the producers didn't have the heart to cover her in the heavy Trill makeup of the Next Generation. Thus, the spots were born!