The point has been made the references are less important to most employers than a simple confirmation of continuous service, etc. for licence, currency (log book) and security pass purposes. This distinction is important as it is more often than not that it is not a traditional job reference, as such, that is required by those carriers that seem to make it a requirement for new joiners.
I am aware of two companies that have played around with not giving references as part of a ploy to keep pilots. One of these is Ryanair (there is only one case involving the other and I don't know the full story). In Ryanair's case this practice does not seem to be widespread (as yet), but seems targetted to certain circumstances and individuals over the past 6 months or so. The recent change in contracts requiring 3 months notice seems to be tied in here, as some pilots who approached FR for "refrences" have been denied them, pending the working out of the 3 months - the purpose of which is, I trust, obvious.
I know of people who have lost job offers as a result of such "administrative cock-ups". What is one to say? Is this just Ryanair being Ryanair, or is it more of the same self-serving behaviour.
P.S. One of the pilots did attempt to point out to the prospective employer what was going on, but to no avail. Another decided to withdraw his three months notice. So you could argue that it is a good pilot retention policy!).
Others have been denied "reference" for as long as possible in questionable circumstances.