I've believed a lie all these years. From he HuffPost:
Timmy, Lassie's second owner in the series, got into a lot of trouble during 7 seasons, including getting trapped in a mine, almost getting shot and being threatened by an escaped elephant, but Timmy himself, actor Jon Provost, revealed in his autobiography that he was never trapped in a well.
Additionally, in 17 season's of the show, the only character to fall down a well was Lassie.
So now, do we have to change it to, "Timmy, did Lassie fall in the well?"
I had to read a bit about Lassie to see what you mean. Apparently he fell into abandoned mine shafts, off cliffs, into rivers, lakes and quicksand, but never into a well. It became a catchphrase after the show's cancellation, but I hadn't heard it.