Much like toddlers, puppies can and will often regress in their training. You know how young children like to test their boundaries and keep you on your toes? Yep, puppies will pull the same tricks, just to see what they can get away with.
You might notice that your (once housebroken) puppy is starting to have accidents again. For some reason, I see this a lot around the 6-10 month mark. The following incidents could trigger a relapse in house training.
An extended visit from another dog. Dogs often mimic each other's behavior, so if one dog has an accident, your dog might think, "Well, if he can pee in the house, I can too!"
You return from vacation to find that your well-behaved puppy is now back to chewing, biting and peeing in the house. Whoever was caring for your dog (a boarder or a friend) might not have implemented the same structure that you have and the result is a dog who now needs a refresher course in doggy 101.
Different work/school hours. The dog can pick up on a change in their routine and they might show their displeasure by acting out/regressing.
Any type of injury, illness or infection. Recently, our dog needed surgery and he had to stay the night in the hospital while he recovered. Although he was completely housebroken, the 3 night hospital stay threw off his schedule and when we finally got him home, he peed everywhere! He was unsure of his new/old routine, so it took another day or so to get him back to our house rules. Or--if your dog gets a bladder infection and accidents are unavoidable, it's not uncommon for the behavior to continue after the infection has healed. You'll need to correct the behavior by offering him a crash course in housebreaking.
Lastly, a relapse in training for no reason whatsoever. Your dog might decide to forget everything you've ever taught them, quite simply because they can. Testing, testing, testing! It's up to YOU to help get them back on track. If you let them get away with naughty behavior once, they'll remember that and they'll try it again. Consistency is key!
Keep in mind, puppies are just like kids. Their first year is filled with experimenting, learning, teaching, testing and exploring. Be patient while they find their way. Expect for things to get chewed and accidents to happen. Remember, great dogs aren't born, they're trained. :)