diff --git a/src/os.rs b/src/os.rs
index acb112c3800..970cfd7219a 100644
--- a/src/os.rs
+++ b/src/os.rs
@@ -26,7 +26,12 @@ use Rng;
 /// - iOS: calls SecRandomCopyBytes as /dev/(u)random is sandboxed.
 /// - PNaCl: calls into the `nacl-irt-random-0.1` IRT interface.
 ///
-/// This does not block.
+/// This usually does not block. On some systems (e.g. FreeBSD, OpenBSD,
+/// Max OS X, and modern Linux) this may block very early in the init
+/// process, if the CSPRNG has not been seeded yet.[1]
+///
+/// [1] See https://www.python.org/dev/peps/pep-0524/ for a more in-depth
+///     discussion.
 pub struct OsRng(imp::OsRng);
 
 impl OsRng {