The WORST thing about adopting a dog is having to give it up because it is not a right fit. I mean that it was abused or neglected to the point that it becomes an unadoptable dog for a person who live in a small apartment complex and is afraid that the dog will bite the neighborhood children. I know. I wrote a hub called "Adopting a Problem Dog Diary" and the former owner is totally INSANE.
To avoid problems go to shelter because they check the dog's behavior for things like biting, snarling, mentally snapping, etc. Check out the picture of the glove on a stick to see what I mean.
Now I have adopted a Chug - he is the BEST thing ever happened to me on four legs. I am very lucky to have Chuggers, (sniff*)