Some end-users like to make their own modifications on their forms (right-click on the form –> setup). Last week I had a customer that was not able to do this trick any more. After some research and a tip from a colleague, I came to the conclusion that this had something to do with overriding the methods tabChange() and tabChanged() on the form.

Solution:
Use the methods pageActivated() / allowPageDeactivate() instead.

Source / More info:
http://dynamics-ax.blogspot.com/2007/06/yellow-lock-in-dynamics-ax-401.html