Your video contains a song you purchased for personal use (e.g., bought on iTunes or in a store) but didn't obtain a commercial license.
You found a video on the Internet and you cannot prove that it's in the public domain.
You are singing words of your favorite copyrighted song and there is copyrighted audio in the background, such as instrumental or karaoke tracks.
You have used content from someone else without permission, but you haven't yet received a copyright notice on your video.
Your video does not provide proper attribution or credits as required by a license.