I found that if you create a custom field of type radio button and create two contexts for it both with the same options list (i had eMail and Phone as the options), and then set Phone as the default in one of the contexts and not set a default in the other, that when you create an issue that has that field on a screen and then swicth to another issue type using the same field but the other context, that you end up not getting the default option selected properly. But if you do not swicth between the issuetypes and start with the one with the deault set it will behave properly. So its a bug that happens only when you are creating an issue -> select issue type A which does not have the default radio button set, and then switch issuetypes you are trying to create (before creating the issue) and the radio button does not refresh to show the default selection.

I've only seen this in 5.0.5 - have not tried any newer versions of Jira.