If you change a bug's Product you'll also have to adapt the Component. The page which is showing the bug's details doesn't give you a hint how to achieve that, though. You simply have to know that you have to press Commit to show the Component selection screen. Suggestion: use JS to show a hint below the Component dropdown telling you that submitting the form will take you to the Component selection screen.

Actually, even better would just be to have the fields auto-update themselves via javascript, which is quite possible, particularly given that we already have a framework for this to happen for custom fields in Bugzilla 3.4.

(In reply to comment #0) > Suggestion: use JS to show a hint below the Component dropdown telling you that > submitting the form will take you to the Component selection screen. Or even better, update the component list, as mentioned in comment 1.