To say you're putting somebody's current job at risk is completely hyperbolic. They've already accepted your offer, if they lose their job (how?) and you hire them, you aren't putting them at risk.

Wait, you'd make a back-channel reference check after making a formal offer and having them accept?

Either this is pointless (because you plan on keeping to the offer and therefore won't act on any information you acquire in the reference call) or you intend on rescinding the offer if you find something bad, in which case it's as dangerous to you as it is unethical.

First, to rescind an offer over a back-channel reference is not just unethical, but it's the kind of thing that, if it gets out, will ruin your company's reputation and yours, personally. (Remember that he has a written offer letter and he can easily get written proof of your rescission. Rescind verbally and all he has to do is show up to work the first day with a video camera.) Turn someone down before the offer phase on a back-channel reference, and he'll probably never know what happened. Do it after you've made a written offer, and you've put your name (and that of your company) in serious danger.

The only time a company can rescind an offer and keep its name intact is when there's a massive, published layoff: e.g. a plant closing. If word gets out that a company backs out of a formal offer on a reference check (you're supposed to check references before you make an offer) then it won't just have trouble hiring, but it will affect client relationships and internal morale. If someone like me is on the other end, I'll fucking make sure that it does.

Try that shit ("I called someone and he didn't like you, so I'm rescinding the offer") on someone like me, and the only way you're getting out safe is to cut a year's salary not to have me destroy you. I'd call every press contact I have and I'd demolish you even if there was nothing to gain for me, for no other reason but this one: God works through people. Your colleagues would log into Valleywag and TechCrunch and see your face. Oh, and since rescinded offers suggest financial troubles and I'd be spreading rumors that similar things had happened to other people, it would raise doubt about your company and they'd fire you for making such a mess. Not wanting this to happen, you'd gladly cut a six-figure go-away check, and if you had this authority I'd begrudgingly let you out safely, but if you didn't, you'd be in for a very awkward conversation with your superiors about why they need to pay a year's salary for someone not to show up, just to cover up your mistake.

You could be a little more slimy and a little more smart about it and, instead of rescinding the offer, hire a person but make his life miserable for 3 months, make sure he can't get anything done, and then fire him if he doesn't quit on his own. Then you're just committing to wasted effort and a bunch of demoralizing conversations with management about systematically fucking over this guy you just hired because you found out that someone didn't like him. Some of your lieutenants will play along and make it look like he failed and not you, and you could possibly get away with it, but others (i.e. anyone ethical) will be disgusted and lose respect for you. Plus, there's inherent risk of drama if you hire someone, never give that person a chance, and fire him 3 months later. Also, if word gets out that you set a new hire up to fail based on a back-channel reference (an unreliable source of information in any case) then the same bad things will happen to you (and your company) as if you'd rescinded the offer: you'll get a deserved terrible reputation.

See, people get fired or turned down for all sorts of unjust reasons and there's so much uncertainty (as with any other breakup, you'll never know who was really at fault) that a TechCrunch or Valleywag would never stoop to post details unless it was CEO-level. That's just regular corporate politics; shit happens, people dust themselves off and walk away from it. On the other hand, if you make a written offer and then rescind it, even if it's a junior engineer, it can find its way to Valleywag (and if someone like me is around to make sure that the right things happen, it probably will unless you pay for it). A bad negotiator might let you off for $10k, but if you try that on someone like me, you're going down for 10-20 times that. And I'm going to make you give up the name of the person giving the bad back-channel and I'm going to hit him personally for whatever I think it's worth to him not to have his employer know that a certain rogue was out speaking on their behalf. (People who give bad references will often pay not to have their more risk-averse employers know that they were out, without permission, speaking for their companies in ways that exposed all parties to lawsuits.) So, whoever you called is not going to like you much. Even if I don't need the money, and even if there's no money to be made (no blood from a stone) I will fuck up your shit, because (again, say it with me) God works through people.

I may be vicious, but if there weren't people like me to clean up after, beat down, and sometimes annihilate the people like you who'd otherwise defecate on the entire world, the whole fucking planet would be 25 feet buried under the shit of people like you.

And what, you think a past coworker is going to contact an employee's current boss and tell him the employee's going to leave? Yeah right. It's implausible that that would happen in any particular case, not implausible that it happened nonzero times in the universe, but again, you're hiring them anyway. Unless it turns out they're horrible to work with, in which case, the employer and the employer's other employees avoided a lot of needless suffering.)

First of all, this brand of justice carries with it a presumed guilt that is not intellectually defensible. You're saying that if someone gets a bad back-channel reference, they deserve to have their current job put at risk by you simply because they're bad people... and they're bad people because your one unreliable source of information said they are. You're like the twitchy guy who lives in his mother's basement, cries when he masturbates, has a paralyzing fear of AA batteries, and flays prostitutes (or women who, to him, just look the part) to death, but thinks it's OK "because they're whores".

You're just a sloppy asshole who doesn't give a shit about what effect he has on other people. That's all there is to it. You don't care if your sloppiness costs someone else his job or if you ruin someone else's career because, well, he's not you.

Word, when put in the wrong place, does fucking get around. If you call someone who's not expecting a call for a reference, you're basically publishing that the person is job searching. It's not that people are malicious or gossipy. (Most people aren't malignant pieces of shit. This might surprise you, but it's true.) It's that once something is presumed to be public knowledge, no one bothers to hide it and it becomes public. Even if your back channel reference isn't obviously connected to the target's current employer, people at his company will find out in a matter of days. It almost always happens that way.

Or you could just not bother because who cares.

"Who cares" sums up your attitude toward people who are not you, and if you don't see why this is a problem, then I don't know what to say. It's sloppy assholes who don't give a fuck about their effects on other people who are giving Silicon Valley and the tech industry the rotten reputation that it is increasingly deserving. Frankly, I think much of the anti-tech backlash is misplaced (I don't think "Google buses" are, fundamentally, a bad thing) but this "it's not wrong if it doesn't hurt me" attitude is exactly what is making the rest of the world hate the tech industry, and we really need to get past this juvenile culture of ethical sloppiness, and start taking responsibility for our effects on the world. I don't know you and have no wish to know you, but I see that you aren't up to the task, so maybe you should make a policy of ensuring that you never have any effect on other people... before you fuck something up in a way that hurts a lot of people who don't deserve it.