Novelty generator
A novelty generator is a pattern that appears to have an unknown fate due to complex feedback loops, for example involving waves of gliders shuttling between perpendicular rakes. Novelty generator patterns fall short of counting as chaotic growth, since the rakes continue to be predictable, and much of their ash generally remains stable.
It has not been proven conclusively that any particular pattern is in fact an infinite novelty generator, since it is always possible that periodicity will spontaneously arise if the simulation is continued far enough. In fact this happens quite regularly. But conversely, it has not been proven that periodicity must spontaneously arise for all such patterns. Bill Gosper, Nick Gotts and others have done extensive experiments along these lines using Golly.