We have Team Systems hooked up to our Active Directory, which is great. It knows who created a work item (or closed it, or edited it) by who is signed in. The dropdown of who to assign things to is prefilled based on who works here. I love it. But recently we removed someone from AD, because she has left the company (to go work in a business owned by one of her family, not that the reason matters to AD or VSTS.) And that led to a problem when I went to save a work item she'd created.
Of course I can fix the "Assigned To" - after all, if I want to see this work item completed, it's pointless to leave it assigned to someone who doesn't work here any more. But "Activated By" - not so much. I can't edit that field and I wouldn't want to anyway, the value has historical meaning.
So, what to do? Neno Loje explains. You change the work item definition so that once something has been validated, that is allowed as a value going forward. You might not want to use it on the Assigned To field, but I sure want it on Activated By.