In this paper it is shown that if
a commutative Banach algebra B with identity has a maximal ideal M whose
algebraic powers M2,M3,⋯ form a descending chain of ideals which never becomes
constant then there exists a discontinuous module derivation from B into a
Banach-B-bimodule. This fact is linked with the known sufficient conditions for every
module derivation from B to be continuous when B is separable. Some examples are
given to demonstrate unusual behaviour in such chains of ideals in particular
situations.