Note that UB is program-global

Author: Havvy

src/behavior-considered-undefined.md

@@ -39,6 +39,12 @@ code.
   * A value in a `char` which is a surrogate or above `char::MAX`.
   * Non-UTF-8 byte sequences in a `str`.
 
+> **Note**: Undefined behavior affects the entire program. For example, calling
+> a function in `C` that exhibits undefined behavior of `C` means your entire
+> program contains undefined behaviour that can also affect the `Rust` code.
+> And vice versa, undefined behavior in `Rust` can cause adverse affects on
+> code executed by any FFI calls to other languages.
+
 [noalias]: http://llvm.org/docs/LangRef.html#noalias
 [pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
 [undef]: http://llvm.org/docs/LangRef.html#undefined-values