In a relationship between two species, could a certain relationship become harmful to both involved species?
Keep in mind, I am looking for a relationship between two species, preferably of the kingdom animalia, although any example will aid my understanding. On an Earth-like planet, the known ecological relationships are predation, competition, mutualism, commensalism, amensalism and parasitism. I would imagine that any sort of relationship that involves both being harmed would not start directly as that relationship; it would evolve to that as a result of a parasitic or amensalistic relationship.
I can think of a few examples, such as a large horse-like animal that feeds on the same shrub as a small insect-like animal, which in turn gets ingested by the horse-like animal, and then attacks the horse-like animal from the inside. The end-game is that both parties are harmed. But I am looking for a relationship that is consistent and each of the parties is entirely aware of the relationship, but continues to engage in it.
If such a relationship could exist, how and why would it evolve to be like that?
As a side note, some of you might by gunning for me to mention neutralism, but I honestly don't care, it is a relationship that describes the lack of a relationship. Doesn't sound like much of a relationship to me. Please, no comments on neutralism.