Although officially leaving after the show's fifth season in 1964-1965, Tim Considine's last appearance as Mike, the eldest son, was actually in the first episode of season 6 (the series' first in color). The episode opened with a brief scene showing Mike and Sally's (Meredith MacRae) wedding. The episode also "launched" Barry Livingston ("Ernie") as the new son. The characters of Mike and Sally were mentioned in the next two episodes, which dealt with Ernie's adoption, and in a subsequent episode when Charley and the boys thought Steve was getting married. Mike was referred to by name one last time in the second 1966-1967 episode, when the gang visited Steve's hometown. After this they were never referred to by name again for the remainder of the series, although Mike was indirectly referred to as, "the first of you" by Steve a few years later.
At Fred MacMurray's insistence, all episodes were filmed out of sequence during the show's entire run using a technique now known as the MacMurray method. MacMurray would do all of his scenes in 65 nonconsecutive days. The cast regulars got haircuts once a week in order to maintain continuity. Guest stars would have to return months later to complete an episode. All kitchen scenes would be done together, then all scenes in the upstairs hallway would be filmed together, etc. This fact was well concealed until Dawn Lyn joined the cast as Dodie. Her upper front teeth grew in irregularly during the entire 1969-70 season, from being barely visible in scenes with MacMurray to being plainly visible in scenes without him. William Frawley never felt comfortable with this method of filming, having grown accustomed to filming I Love Lucy in sequence during its entire run.
Don Grady almost left the show when Tina Cole was cast as Katie. In an interview, Cole revealed that Grady felt she wasn't his type. As it turned out, they eventually fell in love in real life and almost got married not once, but twice.
When William Frawley left the show midway through the 1964-65 season due to declining health, his absence was explained by having Bub move to Ireland to live. He was referred to several times in subsequent episodes that season, then was never mentioned again once the show moved to CBS.
Several years later it was revealed that there were different plans in the works for the character of Robbie when Don Grady left the show. One plan was for Robbie and Katie to divorce and another plan was to have Robbie killed off.
Not only was the first episode of the 1965-1966 season the series' CBS debut, it also was the first episode to be shot in color. Before taking the role of Katie, Tina Cole appeared on a couple of earlier episodes in various roles, always being cast as one of Robbie's girlfriends.
According to one account, Chip was born Richard but as a toddler pronounced his name Chipper. The name stuck and as he got older it was shortened to Chip. If you'll notice his dad regularly called him Chipper.
In the first years 1960 - 1964 Chevrolet was the sponsor and different models were shown throughout each show. Steve Douglas had a 1961 Impala station wagon for the first year. The ending credits also showed Chevrolet photographs which were used in the print ads of the time. In 1964 - 1970 Pontiac was the most noted car sponsor, where Steve had various blue Pontiac Bonneville station wagons while Robbie drove a 1954 Pontiac Star Chief convertible, occasionally nicknamed the "old coffee grinder" by his classmates. In 1970 - 1972, Ford Lincoln Mercury was the sponsor, and Steve had a Mercury Marquis station wagon.