Gil Grissom wasn't the character's original name, William Petersen dubbed him Grissom after astronaut Gus Grissom because he (Petersen) was a fan of the space program. Considering the original surname for the character was Scheinbaum, this was probably an upgrade.
Gil Grissom's otosclerosis was supposed to be a more major storyline and a possible reason for him to retire. However, the show got a lot of letters about the fact that the condition is not that serious when corrected surgically. Hence, the surgery at the end of season 3.
Actor Allusion: In a throwaway line A/V tech Archie mentions that he was going home to watch a variety of surfer movies. The list includes Local Boys, which is a film his actor starred in.
Grissom informs Sara that he loves baseball when she tries to call him out on not liking sports. As any common male reared in the North Side of Chicago tends to be, Petersen is a life-long fan of the Chicago Cubs, usually stopping by Wrigley every season to sing "Take Me Out to the Ballgame", and narrated a documentary about the team.
In 1x08 "Anonymous," Grissom turning to his reflection and "speaking to" the killer is reminiscent of a scene William Petersen had in Manhunter.
Cast the Expert: David Berman, who was a real life coroner before playing 'SuperDave' Phillips, the coroner's assistant.
Corpsing: Speaking of the surgery, Catherine's smirk is probably Marg Helgenberger's own; William Peterson wasn't wearing anything under his hospital gown during the scene. You can infer for yourself the view during Grissom's walking-away part of the scene.
Helgenberger said in an interview once that the original idea was to have "THE END" superimposed over William Petersen's bottom as he walked away in the hospital gown, as a joke. Executives nixed the idea as they thought that was too corny and odd for a crime drama series.
Cowboy Bebop at His Computer: Dr. Raymond Langston is not Grissom's replacement, instead coming in on the bottom rung. Though that doesn't explain why he got to be first in the opening titles, something his character really hasn't earned.
He's a Renaissance Man; he's only a Level 1 CSI (though he works his butt off to get quickly promoted to Level 2) but he used to be a medical doctor (so he assists in autopsy's more than most) and a criminology lecturer (so he occasionally provides a criminal profile). He's under-qualified as a crime scene investigator but he makes up for it with a broad range of skills. Plus he's Laurence Fishburne.
I Am Not Spock: Petersen learned sign language for the episode it was in, but fans have tried speaking to him in sign not realizing he doesn't really know it.
The Other Darrin: Ellie Brass, who was played by two different actresses in 'Ellie' and 'Hollywood Brass' plus 'Bang Bang'. Also Lindsey Willows, but hers is probably a bit more justified since she was a child and growing up fast and the earlier actresses may not have been available.
Marg Helgenberger and Jorja Fox do not voice Catherine or Sara in most of the games.
Real-Life Relative: Marg Helgenberger's now-ex-husband in two episodes (Weeping Willows and one more).
Sort of with Gina Torres and Lawerence Fishburne, only Torres guest starred during season four, six seasons before her husband became the series lead.
Real Life Writes the Plot: Two of the original CSI series' episodes had to be rewritten to accommodate actor absences at the last minute. 'Gum Drops' originally had Grissom as the character believing Cassie was still alive, but William Petersen had to miss the episode due to a death in his family. So Nick was shifted into the lead. More recently, in 'Genetic Disorder', Greg had to be shifted into the lead role intended for Nick because George Eads was attending his father's funeral.
Nick was written out of several Season 14 episodes after George Eads had a row with a female writer.
You Look Familiar: Four of the franchise's primary actors (the ones in the opening credits) have appeared on all 3 series (there may have been more with the one time guest stars, a lot have done at least two) Lawrence Fishburne, David Caruso, Gary Sinise and Carmine Giovinazzo. But only characters Ray Langston, Mac Taylor and Horatio Caine have been on all 3 series since Carmine's guest appearance on the original CSI was 2 years before he began playing his CSI NY character. Word of God says the guest role didn't influence the CSI NY casting.
Marlee Matlin appeared in a role each in CSI and CSI NY.
A.J. Buckley, who plays Adam on CSI New York, appeared in one episode of the original CSI series beforehand. He's the killer.
Fans of both Xena: Warrior Princess and Firefly may recognize Lady Heather as a rogue Amazon and brothel madam, respectively.
What Could Have Been: A fourth CSI series was considered before CSI NY had ratings problems. It would have taken place in either New Orleans (another reason why the series never came to be, as Hurricane Katrina hit around that time) or London (which presumably would have set a new record for number of cast members being a Fake Brit).
CSI Cyber eventually emerged after Miami and NY were cancelled.
Lampshaded in an episode where they become the subject of a Show Within a Show and mention that everything is edited together to make it look faster.
Also lampshaded in a later episode when Hodges mentions that he's convinced everyone that most of the advanced equipment he operates takes twice as long to work as it actually does in order to get more free time.
Note that on many occasions the characters complain about the lack of useful fingerprints, and sometimes the fingerprint collected doesn't match anything on the Omniscient Database.
They subverted this in one episode - a cleanly lifted print in an old case is shown to be fabricated because there was no way that it could have been lifted from the surface claimed (an alligator-skin wallet).