I don't think it would develop without an underlying injury or condition. Was you bird normal before? What's happening now? I think it is safe to say something like this would need a vet diagnosis. Can you provide more info or pictures?
Hi! Let me give details. I bought a pair of GCCs a few days ago. They were given to me in a box. I asked if they could be put into the cage I had in my car but the seller said he didn't have gloves on him. He opened the box slightly for me to look at the birds but it was a quick look and I could see they were alive. I couldn't tell their condition though.
When I arrived home I opened the box, releasing the birds into their cage. I noticed that one of them, although able to climb the cage, fly and eat, rested with one foot held up on the cage bars or in the air. I thought it did this because it was scared but as I watched it more closely I realised something must be wrong with it. I picked it up with gloves and felt along its leg. I couldn't detect any breaks but noticed that one leg was positioned differently to the other. I informed the seller and he claimed there was nothing wrong with its leg when he gave it to me. I know nothing happened to it in the cage. Is it possible for it to have injured itself in the box?
I really couldn't say but if you bought them without being able to see them then anything is possible and it would be difficult to have any recourse.
If he was resting with it like something is wrong then it was probably already injured when he came out of the box. I would be concerned about pain. An x-ray or at least a vet visit would really help you know what's going on. As mentioned already, if it developed or happened recently then something is wrong. If it's an old injury it may be to late to treat but only a professional could really help you.