In the first film, “Star Trek: The Motion Picture,” a horrific transporter accident occurs in the first act. It’s kind of a plot point as the Chief Science Officer (a Vulcan we only briefly meet at Starfleet HQ) is killed along with another poor soul, necessitating Mr. Spock’s return to his seat. It’s fairly graphic, you hear screaming and see deformed humanoid shapes in the transporter “light show” on the ship’s platforms… the transporter technician says “oh no, they’re forming” shortly before what’s left of them is beamed back down to San Francisco. Starfleet ground control then confirms to Kirk and Co that “what made it back didn’t live long.”
Later (like only three or four scenes later), we are told that Dr McCoy doesn’t want to use the transporter to board the ship -likely because of the obvious inherent danger of the device- but is ordered to beam aboard by Kirk. His worry is then played for laughs… as if not an hour ago two people got melted and died. It’s a bizarre shift in tone, only made weirder by the framing of Bones as an old Luddite for being scared to use it.
The danger of the transporter is not really talked about in TOS, outside of accidentally sending folks to the Mirror universe. Wait, I’m just realizing… so, on top of possibly causing untimely (not instant enough for my liking) death and nonconsensual cloning, any old transporter can also accidentally create a portal to fully up-and-running interplanetary fascism. It’s just a dangerous technology all the way down.