I'm building a mx:Panel with two buttons (Login and Register) and two states
(base and "register"). In the "base" state, I set Login Button as the default
button. In the "register" state, I set Register Button as the default button.
The base state has two text input fields: userid and password. The "register"
state has userid and password plus other fields for personal data. All the
fields are checked by Validators before submitting to server. A mx:LinkButton
is used to change states.

When there were no data entered and I submitted the form, I got warning from
the Validators. After I did it for both states and if I pressed the Enter key
again, the state is changed just like clicking the LinkButton. It should
display warnings, not causing state change. It seems the LinkButton becomes the
defaultButton.

When I took away the defaultButton settings, the problem goes away.

I'm posting this to see if others have seen the same problem.
I hope I did something wrong, but it looks like a bug. I still want to use the
defaultButton, is there any workaround?

The defaultButton in a container can change if focus moves to another button.
Check that you are not moving to the link button somehow after pressing enter
(set the linkbutton:tabEnabled =false), which then becomes the defaultButton.

I tried tabEnable="false" with LinkButton, but still get the same behave.
tabEnable="false" only causes tab to skip the LinkButton. How to remove the
focus from LinkButton automatically after it is clicked?

Can you post some application code so we can see what you're doing? It would be a lot easier to help out with specific code.