Alias Smith and Jones (1971-73) was one.
Rod Taylor's buddy-boyfriend in The Hell with Heroes (1968), and he was also been making the rounds of tv dramas, as well as doing a few comedies (such as Gidget).
Outlaws trying to go straight, they criss-cross the Old West, getting involved with people's problems along the way. Thankfully, few of those problems involved old girlfriends or current flames, and many involved rescuing each other from cliffhanging danger.
But they only filmed 18 episodes together. On December 31, 1971, Pete Duel, who had been depressed and drinking heavily, committed suicide.
Dark Shadows, as Hannibal #2. Gay fans were outraged -- how could they replace a boyfriend so cavalierly?
But the program managed to keep going on through the end of the 1971-72 season and halfway through the 1972-73 before being cancelled.