critical mass n.
In physics, the minimum amount of fissionable material
required to sustain a chain reaction. Of a software product,
describes a condition of the software such that fixing one bug
introduces one plus epsilon bugs.
(This malady has many causes: creeping featurism, ports to too
many disparate environments, poor initial design, etc.) When
software achieves critical mass, it can never be fixed; it can
only be discarded and rewritten.