any ideas on what could be wrong with my dog?

I have a cairn terrier mix. We got him from the shelter and they said he was a cairn terrier but he is much smaller than all other cairns i know. Anyways he is around 8 or 9 years old. Sometimes, just randomly, one of his back legs will fall to one side. He always recovers and is able to stand again, but it is odd. He doesnt seem to have any pain when it happens, but i could be wrong. These usually happen when he is playing or gets excited. I dont think it is too serious, but any ideas on what it could be?

You should take him to the vet to get him checked out. It sounds like he is losing feeling to the leg. It can be anything from a slipped disc to a tick bite or cancer. He should not be losing feelings to his leg even if he is excited or playing.

My dog is old and does that also. It can be arthritis.

It definitely sounds like a neurological problem. I would take him to the vet for tests.