On many occasions I've heard people describe their dog's behavior as "unpredictable". Trainers know that dogs are not only predictable in their actions, but highly predictable. Identifying these triggers is something that should be left to the assessment of professionals, as things are so often misdiagnosed by pet owners.
If I had to give an example of a situation where I thought "unpredictable behavior" would be prevalent there is one scenario that comes to mind immediately: the re-introduction of litter mates as adults. I am fortunate enough to be very good friends with people who have litter mates of two of my dogs. Whenever they have come over to visit we have to take things very slowly. It was crazy to see the reactions of both dogs. My dogs are used to many other dogs being in my home, but as soon as the littermate of my Staghound entered our house, my dog Tom was acting very peculiar. It was a very strange form of recognition, but it was not exactly a happy one. I had never seen him care so much about another dog entering the home. He hadn't even made visual contact with his brother yet, but even from the other room, he knew that something was up and this was not just another dog. We had to keep the dogs on leash for a few days before we eventually trusted them off leash together. Even still, they never really clicked, as both of them would do with just about any other random dog that they met. They both tolerated the presence of each other, while still having this odd familiarity between the two of them. It was weird and uncomfortable to say the least.
Rain, my Canis Panther, has been reunited once with one of her litter mates, but they were both still puppies at the time. This has been the first time they have been together as adults and just like with my Staghound, I took it very slowly. We did a lot of side-by-side walking and a lot of just doing obedience exercises around each other. When we got back from a long run, I could see that both dogs had finally smoothed out with the presence of one another and acted like they wanted to play. Of course both dogs had their E-collars on so I could give them information if they made any poor choices. It is nice that they had this chance to play with one another and that they both enjoyed it, however, I would not trust the two of them loose together unsupervised, ever. There will always be something awkward and unpredictable about this type of interaction.