Note that stage1 is built using two different versions

src/building/bootstrapping.md

@@ -209,6 +209,9 @@ The `rustc` generated by the stage0 compiler is linked to the freshly-built
 so that `rustc` can use features added to std immediately after their addition,
 without need for them to get into the downloaded beta.
 
+Note this is different from any other Rust program: stage1 `rustc`
+is built by the _beta_ compiler, but using the _master_ version of libstd!
+
 The only time `rustc` uses `cfg(bootstrap)` is when it adds internal lints
 that use diagnostic items. This happens very rarely.