Account transfers are easy and instant, so there are rarely issues, but they do occasionally occur. The two most common are when transferring a newly registered domain, or when the account code is not correct.
When finishing an internal transfer, you may get a notice that says Error: Cannot Transfer (Domain Name). Domain was Registered in the Past 5 Days. This is an anti-fraud measure that we cannot bypass. You simply need to wait until 5 days after the registration date to complete the account transfer.
The second error you might see is the incorrect account code. If you see The Account Code is Invalid then you do not have the correct or complete code.