Sometimes we're wrong in science.
Sometimes we're very wrong.
Scientists trying to sequence two species of lichen discovered they were genetically identical. Both the photosynthetic algae and its fungal symbiote were identical.
One of them was toxic. The other wasn't. So, something else had to be going on - and the something turned out to be a second fungal symbiote specialized for defense.
Scientists are now trying to find out whether all lichens are, in fact, threesomes...or only some.