When Stacy Manning is seen sitting with Bill Bixby the first night in the shack, the moment of her wafting away Bixby's cigar smoke was entirely ad-libbed. Stacy Manning couldn't stand the secondhand smoke.
When Donovan loses his poker hand to the banker, he mentions "three deuces beats aces over eights". In poker, two pair (aces and eights) is sometimes called the Dead Man's Hand. According to legend, this was the poker hand held by Wild Bill Hickok when he was shot dead.
During the scene of Theodore and Amos crossing above the alleyway to the bank, Don Knotts was genuinely nervous while being so high up. Tim Conway fooling around on the ladder in the scene did not help matters.
The $87k+ gold nugget at $15/oz (based on the gold price given by Don Knotts in the film) would be worth more than $9M in 2019 gold price value.