All-Star Cast: Twice, in "Separation" in season 3, and in "The Field" in season 6.
Missing Episode: 10 episodes. 2 from season 5, and 8 from season 6 to be more precise. You may have seen them in syndication, however.
Schedule Slip: In season 3, the three remaining episodes were sheleved in favor of the first season of The Charmings. They were shown in May. Also in season 4, two episodes were shelved, and didn't air until season 5. Finally, there were 10 unaired episodes (2 from season 5, and 8 from season 6).
Screwed by the Network: Though never a top 30 hit, it won its timeslot, many times, and was brought back for a 4th season in 1987, due to fan protests. But ABC would soon begin tinkering with their Friday night schedule, by making sure it was dominated by Miller-Boyett programs. It began in 1987 with Full House as the lead-in for the show. Then a year later, in 1988, ABC moved Perfect Strangers moved to Friday nights. Finally, a year later, ABC moved Mr. Belvedere to Saturday nights, to make way for Family Matters. Only Just The Ten Of Us remained. By the end of the 1989-1990 television season, Mr. Belvedere and Just The Ten of Us were cancelled, and there were no more non-Miller-Boyett shows in Friday nights for the time being.