Never seen anything like this myself, but I have heard stories. Mix-ups have happened, particularly with cable. Here's one:
https://nationalpost.com/news/canada/porn-movie-interrupts-morning-news-broadcast-at-hamilton-tv-station"Viewers watching a TV station in Hamilton, Ont., Friday morning noticed the newscasters onscreen were wearing less clothing than normal.
At 9:30 a.m., the CHCH broadcast was replaced with a few minutes of hardcore gay pornography instead of the news, prompting outrage from many viewers and a slew of apologies from the station."