(at around 1h 6 mins) While Eddie is holed up at Coit Tower trying to find Cletus, several helicopters pass by looking for Venom and Carnage. This was actually not planned; the helicopters flying by were actually part of the shoot for The Matrix Resurrections (2021), which was filming in the same part of San Francisco as Let There Be Carnage during their shoots. Instead of waiting for a clearer shot, Andy Serkis thought it would make sense for helicopters to be roving around during the scene and left it, adding additional dialogue by Tom Hardy in ADR afterward to mention them.
(at around 15 mins) When Brock enters Mrs. Chen's shop, Venom arranges a stack of magazines for her. On the cover of those magazines is Marvel writer Stan Lee.
When Eddie and Venom are arguing with each other in his apartment, Tom Hardy recorded lines as Venom which were played back (in an earpiece Hardy wore) by a sound operator. This technique let Hardy improvise his dialogue as an exasperated Eddie.
Andy Serkis only agreed to direct the film on the proviso that he would be allowed to film the majority of it in his native UK and do all the post-production work there too, because he didn't want to be away from his family for long periods. Unknown to him, Sony was planning on doing this anyway, and therefore were looking for a UK based director to oversee the production. As it happened, this proved to be very useful, as during the worst part of the COVID-19 pandemic, international air traffic was temporarily closed down, but production could still continue.
(at around 15 mins) Venom greets Mrs. Chen with "Wan shang hao!" or "Good evening" in Mandarin.