Force of Nature

In this episode a Federation wide speed limit is put in place because of the harmful effects traveling at high warp has on subspace. While it is never mentioned in any of the TV series or movies it was mentioned in a book that the new class 9 warp drive being tested in the new Intrepid-class star-ships, such as the U.S.S. Voyager, had been designed to not have those harmful effects on subspace. So after the new class 9 warp drive tests were successful all Starfleet vessels had their warp cores replaced with new ones with similar modifications to the class 9 warp drive, which is why the Federation wide speed limit is never brought up again.

Data's cat, Spot, is a male in seasons 4-6, but is female in season 7.

Edited from Star Trek: The Next Generation: Suddenly Human (1990)