You could end up with a blank value if the backing bean's SelectItem list changes and the selection value is no longer in the list. Although that can also lead to validation errors.
A list mismatch can occur for several reasons. One of them would be if you tried to use Request scope on the backing bean. Request scope in
JSF isn't as usable as it is in straight
J2EE.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.