It's pretty common, well not that common but I do see this from time to time when guitars go out of tune in the middle of an song. Well this is because new strings do tend to go out of tune when you play. And if you swap out strings before every show like Tommy Emmanuel and any other guitarist do before they go on stage then this will become a problem if not dealt with properly.
But it's also something that we common folks have to deal with when we change strings. So the advice we get here is something that we all should take to heart.
If anyone should know what they are talking about when it comes to guitars and guitar strings Tommy should know because he have been around guitars and playing for the most part of his life.