How common is reference/citation padding?
Not in my experience, but absence of evidence is not evidence of absence. This might require a genuine literature search using various keywords to give a quantified estimate for particular fields. I read citations quite closely in my field and I have only rarely seen this practice (&lt;1%). Much more common is slightly misrepresenting what the other authors said, or referencing something a reviewer might suspect the author hasn't read, like a tangentially related book used for a brief aside. And, of course, quasi-related self-citations.