Baharein Phir Bhi Aayengi by

In 1966 Gurudutt was working on a Tragic Lovestory - Baharein Phir Bhi Aayengi. Sachin Dev Burman was supposed to compose the songs for the film. Burman Da had just seen Bridge On The River Kwai. He composed the title track based on the whistling tune from the film, sung by Mohd. Rafi.
But SDB's health was in shambles, he suffered a stroke and was hospitalised. Dutt was in a hurry to make the film, so O.P. Nayyar was hired to compose the songs for the film, Nayyar as the narrative goes refused to retain the song earlier composed by S.D. Burman and created a new title song. As the fate would have it, Dutt committed suicide while Baharein was still midway and he had to be replaced by Dharmendra.
Meanwhile, SDB came back and started composing for Jewel Thief (1967), where he reused the same tune for Dev Saahab's intro song, but this time with Kishore Kumar. The song went on to become one of the most successful songs of the year.
Presenting the very rare Mohd. Rafi version here -