If a single member from one of the parties in the coalition decides to transfer to a party in the opposition, would that cause the coalition to become invalidated, as there would no longer be a majority? Or, because it started with a majority and no whole party left, perhaps the government would still be functional.
It would not immediately invalidate the coalition, but it could prevent the government from doing any business without negotiation with the opposition. Measures with cross party support could still pass. It could also result in the opposition passing a motion of no confidence, which would invalidate the coalition.
As noted in the comments, this is not really 1 MK bringing down the but 61 MKs bringing down the government.
In fact the 20 Knesset survived, and the Coalition was strengthened by another minor party, Yisrael Beiteinu, joining.
An analogous situation which did result in the collapse of a government arose in the UK in 1979. One labour MP was mortally ill. Despite his determination to vote, his party leader told him not to risk his health by travelling to parliament. As a result, the Labour party lot a vote of confidence by one vote, causing the governing coalition to collapse and a general election to be held. (The MP, Alfred Broughton, died a few days later)