In the Jimmy, Jerry goes to his dentist, and while not verbally confirmed, you can see his shirt was tucked into his pants before he went under anesthesia and is untucked afterwards (implying he was assaulted by his dentist and the assistant while under). He has a conversation with Elaine about being "damaged goods."
[SPOILER for S5E4 “The Sniffing Accountant”] the characters suspect an accountant is a cocaine addict because he sniffs a lot, but he ends up just having an allergy to the material of a character’s sweater.
Jerry throws a bucket of ice water over a balcony on to a cage of doves in S06E01. You can hear the doves screeching and find out they died the next morning
Season 9 Episode 6 revolves around dark humor that plays on the idea of date rape, where Jerry drugs a woman and plays with her toy collection while she's unconscious.
Not a clown per se, but in one episode Kramer has a ventriloquist doll who at the end of the episode who appears to be alive and running around at night in Jerry's apartment. This takes place in season 8, episode 8.
Also a character called Crazy Joe Davolva dressed up as a clown and acts threatening toward a couple of people, including Kramer who is scared of clowns. This takes place in season 4, episode 9 called the Opera
The black and white cookie! Jerry has a 7 year record of not v*ing and he eats the cookie, then says "here goes 7 years down the toilet" and walks off. Nothing is seen or heard. (I may have small details wrong but you never witness any v*)
Right at the end, someone is about to jump off a bridge, but a lawyer wearing a cape saves her (it's a reference to a joke jerry made earlier in the episode, so it's meant to be comedic)
Elaine dates Joe Davola without knowing about his mental problems; she walks into his apartment and finds the walls covered with pictures he has secretly taken of her. Davola acts very sinister and creepy, then tries to trap Elaine in the apartment; she sprays Binaca in his eyes and escapes. Played for laughs, but surprisingly tense for a Seinfeld episode.
Elaine has a disagreement about abortion rights with Poppy; later she mentions the subject to a guy she is dating. There's no mention of an abortion actually being performed.
No derogatory slurs, but Jerry and George are very uncomfortable with the idea of anyone possibly thinking they might be gay or effiminate, though they usually follow up by saying “Not that there’s anything wrong with that…”
There’s an episode (S5E10) where Jerry brings over to Elaine’s apartment a “cigar store Indian” statue and makes insensitive stereotypical jokes, not knowing that one of Elaine’s friends who’s over at the apartment is indigenous and she is rightfully extremely offended.
There's an episode based around George being mixed up with the leader of the Nazi's and he reads an excerpt from the leaders book which is very anti-Semitic but Jerry Seinfeld is Jewish and most episodes are safe in that manner
When Elaine finds a list of excuses Seinfeld came up with to get out of going to a game with someone, she pitches that Seinfeld tells the person that he’s been diagnosed with “multiple personality” (an outdated term for DID/OSD.) It’s not dwelt on but it’s still there
all characters, but mainly the men, sexually objectify the people they’re attracted to. George basically sees women as porn/objects, and never treats them with humanity or dignity. Kramer and Jerry can get lost in the moment occasionally but ultimately they usually treat women like human beings.