The Other Darrin: Kimberley Duncan is played by different actors from show to show, and each one has a different take. For example, one acts a lot like an Excited Kids' Show Host, another tends to say "Super!" a lot.
What Could Have Been: Billy Idol was originally slated to play the T-1000, but a serious motorcycle accident in 1990 left him incapacitated and unable to work. James Cameron also considered casting Michael Biehn, who played Kyle Reese in the first film, as the T-1000, and would've wrote around it by saying that Skynet managed to get Kyle's DNA and used it for the T-1000, but the idea was binned when he thought audiences would get confused by this.
Fourth movie: Halcyon (along with the 3rd film had Warner in the US, Columbia worldwide)
Fifth movie: Annapurna Pictures
This also leads to the first two being released in home video by a plethora of companies.
Defictionalization: Of a very scary sort. The British Ministry of Defence actually operates a satellite network used to coordinate unmanned vehicles - including "Hunter Killer drones" - called SkyNet.
The US Air Force has a unit readiness tracking system called, I shit you not, SkyNet. During exercises, announcements come over the loudspeakers for group commanders to "update numbers in SkyNet".
There is a company called Cyberdyne that is working on exoskeletons. Based in Japan. The version it's getting the most attention for is called the HAL 5. Anyone else see the three problems with this?
If parts of T4 gave you nightmares then just skip this link. At least there's no... teeth
News 4 in Tucson, Arizona has a weather, traffic, and safety observation network called Skynet:  They have billboards for it all over the city and it's a bit unsettling. (Especially when in the Sarah Connor chronicles series the Los Angles traffic system was originally destined to be the "nervous system" of Skynet.)
Arnold also had to get back into bodybuilder-level shape to play the T-850 in the third one.
The Other Darrin: John Connor has been portrayed by numerous actors; with 7 actors playing him in 4 movies and a TV show. Sarah Connor was recast for the T:SCC series as well, with Lena Heady taking the iconic role from Linda Hamilton and ironically sharing the same initials.
T4 also has a new Kate Brewster and Kyle Reese; understandable and necessary in the latter case, since he's several years younger than in the original film.
Reality Subtext: Remember the orderly that Sarah brutalizes during her breakout? The one who beat her so she could be restrained? The actor kept pulling that blow too much, and Linda Hamilton badly bruised her knees on the hard floor with each take. She took out her frustration on the poor guy. Also Enforced Method Acting.
Sequel Gap: 7, 11 and 6 years between installments.