I've been complimented in the past on how I'm able to keep to the key, so I was shocked recently to discover that I drifted on one song. I have it on very good authority though that it's very common, and even the most experienced singers can do it - Hedy West has, for example.

Maybe it might be worth finding out if there's a specific point in a song where you tend to drift, and that might give clues as to a pattern? and/or work with it with a voice teacher.

I'll be watching this thread to see what I can learn too...