A friend of mine is idly looking at new job opportunities, and drew my attention to a posting which included a requirement I haven’t seen before: when asking for references, it specified that one reference must be from a current manager or supervisor. There are two major flaws in this requirement, and both of them could get an employer into trouble:
1. This is discriminatory. It limits job eligibility to those who are currently employed, which is not a bona fide requirement for the role. There are many people who could be perfectly qualified for the position but, for one reason or another, not have a supervisor they can ask; more likely, a good candidate might be out of a job due to being on parental leave, having had an injury which put them onto disability, having quit a previous job due to a toxic environment. There’s an underlying assumption, with this requirement, that anybody worth employing for this job is already employed by somebody else. This is, in a nutshell, discrimination on the grounds of employment status.
2. In order to get a reference from a current manager, two conditions need to be met: a) your current manager has to like you enough to give you one despite the fact that you’re planning to leave them; and b) you have to be willing to jump ship from an employer which likes you enough to give you a reference. Either way, it’s kind of like looking for a new spouse among adulterers – chances are, you’re going to be cheated on too. Do you really want to hire a person who’s willing to jump ship from a happy place? What sort of corporate culture does that imply?
If you manage to get good, qualified job candidates in response to your posting, asking them to provide recent references (i.e., relating to their work in the past two years) is useful information. Expecting somebody to produce a reference from their current employer is going to limit your choice of candidates unnecessarily, and possibly lead to some trouble.

Hmm, I can envision another scenario: an awful employee who the manager hates but isn’t willing or able to fire; giving them a glowing reference would sure be one way to get them gone. =)