This has just happened in our Council. A Councillor became upset, but not because of the Town Council. She stood for County Council as well and got some unpleasant comments to her online post of her proposed policies. In the heat of the moment decided to leave the Town Council as well. Fortunately she sent the resignation to the Clerk but not to the Chair. After calming down she has decided to stay on the Town Council. There really should be a cooling off period after sending a resignation, even if it's just a couple of days.