A boy becomes possessed by a supernatural being. When a part of this supernatural being is burnt with a flamethrower, the boy is thrown onto the ground and starts to have a seizure.

A character has a seizure and injures their face on a desk - it is not a medical seizure, but is rather a coercive supernatural event. However, other characters in the scene believe it to be a seizure

child has a seizure at the end, and a previous seizure is discussed.

A character has heart problems and falls to the ground spasming in a seizure-like manner.

Not really. Beverly Marsh floats and her eyes turn white but shes not having a seizure. And Pennywise in his transformation from Georgie to himself shakes weirdly but I dont think thats a seizure.

Many seizures, including grand mal, are shown on screen. One episode has a man who has a severe seizure and while post-ictal is violent.

One character suffers from at least one seizure during the show because he had untreated encephalitis

Jojen Reed has seizures whilst seeing visions. This happens multiple times several episodes. At one point he has a seizure whilst kidnapped.

season 3 episodes 1 and 3 ( or maybe 2) both have seizure scenes

Scully has a fake seizure for a case they do

Rue overdoses and has a seizure

At one point a character's robotic heart can be pulled out, causing them to twitch in a sort of seizure-like way

The effects of an MRI scan on Eddie and the symbiote cause him to convulse violently, not unlike someone having a seizure.

S1E7 about halfway through main character has a seizure due to poisoning

There is a stroke due to hysteria, but no seizure.

SPOILERS: A character appears to have a seizure (or something similar) onscreen after being injected with too much morphine.

A recent statement-giver (ep. 155) has a near-death experience following a seizure.

At one point, the son, Samuel has a convulsion in the car.

additionally, all the "possessions" look like the start of a seizure.

Drug OD seizure

Someone has a convincing fake seizure Later a guard has a stroke that involves seizing

In season 2, Carl has a seizure after he is shot.

Danny looks like he’s having a seizure.

Towards the beginning, blood appears in his water just before he starts shaking and coughing up blood.

In Episode 8 someone has a seizure

No, but there is a possession scene which includes back arching/labored breathing/choking, so this might be triggering from some

At the end of season one/begginning of season two, a character has a seizure after being beaten and passes away.

Cain goes into spasms when the alien larva exits his body.

In Episode 6, Sabrina is almost possessed and it looks like a grand mal seizure