printf can/does have side effects. Perhaps one of the parameters to the call is obscured by a macro, which has some other effect. Or perhaps stdout was redirected and the printf was required to prevent line buffering from hanging a downstream consumer. The point is, if it seems very unlikely that such and such could cause a problem, it probably isnt the true cause. (Printf itself isn't the fix, it is just involved in the fix)
interesting, thanks for the explanation.
printf() introduces delay that is often enough to hide / "fix" race conditions.