I'm jumping into this game late but I came here because I am experiencing some interesting negative behaviors from a dog I was assured is perfectly well-behaved in every way. I was hoping others had experienced this. I think I am going to simply let the owners know that their dog exhibited X,Y and Z behaviors, most likely due to anxiety/new circumstances, and that future sitters would love to know about it so they can be prepared. Trying to phrase it more as a positive thing ("Daisy was a real peach! She did, however, show some behaviors I thought you should be aware of. She was aggressively possessive of the toys she brought with her, snapping at me and growling when I touched her belly or neck, pulling hard on the leash and lunging at other dogs/people. She's a real sweetheart and I'm sure these are just behaviors that stem from being in a new situation, but I thought yu'd like to know how her adjustment manifests so you can prepare future sitters!"

Hopefully this way it sounds like I am not blaming anyone for her behavior, and just providing another service. Hope this helps out the next person in this position!