Character mistake: When Henry is describing the fishing reel he bought for his girlfriend, he said, "with jeweled escarpment" actually it is a 'jeweled esCAPEment" not esCARPment. An escarpment is a long slope off a plateau. An escapement is a latch/release mechanism that you would find on a device like a fishing reel.
Not with My Sister You Don't - S1-E19
Character mistake: Howard Borden goes to Bob's office and says it's the first time he's been there. In fact, he had come to the office to address Bob's "fear of flying" group in the first episode.
Character mistake: When Mrs. Slocombe volunteers to sing while Captain Peacock plays piano, Mr. Humphries responds that he didn't know she could sing, even though the department staff has been involved in many musical numbers over the series.