Getting Your Warnings Back (Resetting All Dialogs)
One thing I love about Photoshop is that a number of warning dialogs have a magical checkbox that says "Don't show this dialog again" (or something along those lines). However, if you later decide you want these warning dialogs put back into play (this is especially helpful if you're training someone new on your computer), you can have them become active again. Just go under the Photoshop menu (PC: Edit menu), under Preferences, and choose General. In the General section of the Preferences dialog, click on the button at the bottom of the dialog named Reset All Warning Dialogs.