Pornographic satellite channel Xplicit XXX has been fined £50,000 for broadcasting hard-core sex scenes before the 9pm watershed.
Ofcom was concerned children could have been watching the broadcast
The channel is usually only available to subscribers after 2200 BST, but can run promotional videos earlier if they are not sexually explicit.
The company said it had accidentally transmitted the wrong tape, said BBC media correspondent Torin Douglas.
Ofcom said the channel had been "excessively complacent".
It is the first penalty imposed by media watchdog Ofcom since is started work at the beginning of the year.
Xplicit XXX told the regulator its usual scheduler was on holiday when the promotional tape was run as early as 2030 BST by another member of staff.
It said the employee had mixed up two tapes in identical containers, said our correspondent.
He said Ofcom had been concerned a child could have seen the material.
Ofcom had been considering imposing a six-figure fine, but limited it because the company was making a loss and no viewers had complained, he said.