In all of my trips to Costa Rica this was the first time I observed this behavior. Two Toucans were sitting on a branch together in the forest canopy when another toucan appeared and landed on a lower perch. Immediately one of the toucans above flew down and he started fighting with the new arrival. They locked bills and fought beside each other until the one toucan flipped upside down on the branch while hanging and spinning the new arrival beak to beak. After this struggle the intruder flew off completely out of sight while the other toucan seemed to stand guard on that branch until he was sure the other wasn't coming back and he flew back up to the other toucan still waiting on the higher perch in the canopy.
In this photo the intruder was being hung from the other toucan's beak and you can see that the tip of his beak is extremely close to the intruders eye. Despite its size, the toucan's beak is very light and is composed of bone struts filled with spongy tissue of keratin between them. The bill also has forward-facing serrations which resemble teeth. I don't know if the eye was injured because he flew off so quickly once he managed to break loose from the others grip...but I think he was probably fine and the only thing injured may have been his pride.