If you have installed any apps that uses your camera, then better sort them out. They may be the culprit that causes the warning message Camera Failed in your Samsung Galaxy phone. I don't know if it happens in other Android devices, but still this will serve as a precaution.
Camera Failed warning message normally happens when an app doesn't handle properly the camera, or never release its use in it after you close that app. In my case, I figured out that the security app I installed in my Samsung Galaxy W bugs my camera. This security app has a feature which you can remotely take photos on your phone, just using a web browser. I was so glad that it has this feature, not until when I am going to take a photo myself on the phone, it shows the warning message Camera Failed. It resolves by rebooting the phone, but after a while, its back.
I searched for some solutions in the web, and finally read about a post telling the users that a certain photo capture app is the culprit for that error message. It just gave me the idea that I should sort out those apps that uses my phone's camera. And there it is... I figured out that one of my apps installed that uses the camera is causing the error. The irony is, it is a security-featured app. It's a popular free feature-packed app in the market that showcase abilities on tracking your lost android phone. In fairness, it really do what it has advertised, but it's still buggy, and it includes it's handling of camera. Anyway, I hope I was able to help you and gave you an idea on how to solve the Camera Failed problem.
You may also follow the steps in this other blog. It is very helpful if the error still persist after removing those apps. I suggest that you better bring it to the authorized support center of your phone if the steps on the blog were all exhausted, and especially if your phone is still under warranty.
This is it and enjoy your photo shoot! :)
Another way to check why the Camera Failed message appears is if the app that uses the camera was closed properly. You should also be aware that Android apps closes themselves differently. Some have Quit or Exit menus accessible via Options button/touch panel or through the app's menu toolbar to completely close it. Others use the Back button/touch panel to close them. Either way, this will ensure that it will stop using your phone's resources once you stop using it. Never use the Home button to clear it out from your screen as this only minimizes it as it continues to run in the background, still using your phone's resources and hardware.
Also check if a certain app that uses your phone's camera is running as a service in the phone. They are classified as service apps if they automatically start at boot, and doing some monitoring or scanning activities. If there are options in the app that disables them from running as a service then activate it. Be sure you do this on apps you downloaded from software sources.