Sprouts and parasites
Mitchell Riley was investigating interactions between a perpendicular p20 backrake and dirty Schickoidal puffer. One particular arrangement initially seems rather inconspicuous, but actually harbours an incredible surprise after approximately 1.6 million generations. Shown below is a snapshot after 2 097 152 generations:
What is that unusual oblique protrusion? Mitchell initially thought that it may be a switch engine, but it is clear from the angle that it is not: switch engines travel diagonally, not obliquely. It is, in fact, something that seldom occurs naturally. The gliders from the east-directed Schickoidal puffer have partially annihilated the glider stream from the north-directed backrake. A northeast-directed glider has collided with this stream, causing a messy reaction. It just so happens that the reaction generates another glider, which hits the stream of gliders again, triggering the reaction again, ad infinitum. (In this case, though, outside influence causes the 'parasite' to be terminated at circa 2 million generations.)
He discovered three more parasites, this time on a single p8 backrake. More interestingly, Bill Gosper noticed that two antiparallel backrakes can allow these parasites to reproduce, leading to an entire profusion of parasites on each rake. However, the number of parasites is limited, as they each consume a nonzero proportion of the rake's gliders.