In 1992, the BBC banned the third season episode “The High Ground” from airing because Data spoke a throw-away line referencing terrorist activity in Northern Ireland.

TOS/TMS Crossovers In TNG

While TNG is supposed to take place 78 years after the original series, there have been occasions in which various characters and actors, who appeared in either the original series or the movies, reappeared on TNG. Among these are....

"Encounter at Farpoint" - DeForest Kelley makes an appearance as an unnamed admiral who hates transporters and notices that Data, with all that knowledge, doesn't have pointed ears.

"The Naked Now" - The records from the original Enterprise ("The Naked Time") were used to combat a microbe that causes drunk-like effects.

"Symbiosis" - Merritt Butrick was T'Jon in "Symbiosis", and, of course, appeared as Kirk's son David Marcus in Star Trek II: The Wrath of Khan and Star Trek III: The Search For Spock. (After his appearance in TNG, Merritt Butrick died due to complications of AIDS)

"Sarek" - Mark Lenard makes an appearance as Sarek.

"Unification" - Leonard Nimoy makes an appearance as Spock and Mark Lenard as Sarek. Malachi Throne, who plays Romulan Senator Pardek in "Unification", was involved in TOS's "The Managerie" as Commodore Jose Mendez.

"Relics" - Scotty modified the transporter so that the transporter buffer is recycled and the rematerialization subroutine is disabled so that the two survivors of the Jenolin could survive while waiting for a rescue. 75 years later, the Enterprise reactivates the transporter, and Scotty is the lone survivor, without aging.

Kirk has been mentioned in "The Naked Now", "Unification II", and "Relics".

Diana Muldaur, a guest star on TOS episodes "Return To Tomorrow" as Dr. Ann Mulhall and "Is There In Truth No Beauty" as Dr. Miranda Jones, appeared on a regular basis as Dr. Pulaski during the second season of TNG.

Majel Barrett, who played Number One ("The Cage") and Doctor/Nurse Chapel, is used as the TNG computer voice, and has appeared as Deanna Troi's mother, Lwaxana Troi.