I agree that it is a serious error as it may indicate semantic inaccuracy. However, my justification for putting it as warning is that we don't actually stop our execution if we encounter this, that is, we are not throwing an exception. Maybe we should? Although their may be cases where the file that has been "moved" is actually an empty file, in which case it is not an error, does not justify throwing an exception or an ERROR tag. 

That's why i used a warning.