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.
George drives over several pigeons, we do not see them get hit by the car, but feathers fly around the car as they are hit.
In the same episode he also accidentaly drives over a squirrel to save some pigeons, the squirrel is taken to the hospital and taken care off.
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.
There are brief mentions of cheating occassionally throughout the show but these are the big ones off my head
s3 ep15: Jerry starts a relationship with his neighbor's girlfriend while he's in a coma
s3 ep20: a married woman has an affair with George
s4 ep20: Jerry's stand-up bit is about affairs
s7 ep13/14: George tries to get a date and hides it from his fiance
s8 ep13: George claims to have had sex with his coworker's wife to piss him off (though it's untrue)
s9 ep12: Newman has an affair with the super's wife
s9 ep14: Elaine dates a married guy
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*)
Throughout multiple episodes Joe Davola is shown to have mental health issues and is on meds for it. He also threatens to hurt Jerry and indirectly threatens Elaine.
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.
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
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
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.