We called his previous manager (not his current one), one of his references, to ask him about his experience with the candidate. His manager spoke very highly of his past direct report, we were quite happy with the outcome of the call.
Except his manager then proceeded to counter-offer the guy with 240, once he realized he was on the market, and the guy was like "Hey guys, you're awesome, the mission is meaningful and all that, but I got a family, I HAVE to take the extra 60k, sorry". That was one day away from him signing. We couldn't afford to counter-offer, so that's that.
Lesson: the engineering hiring climate is vicious right now, you might not want ask for references from your star candidates to avoid getting a last second poaching. Backdoor references in general are a lot better, but you still risk disclosing information that will lead to them getting poached.