diff --git a/book/en/src/by-example/app.md b/book/en/src/by-example/app.md
index e0f4f941ec..9a073ac4cf 100644
--- a/book/en/src/by-example/app.md
+++ b/book/en/src/by-example/app.md
@@ -77,6 +77,9 @@ references that are safe to access.
 
 The example below shows that `idle` runs after `init`.
 
+**Note:** The `loop {}` in idle cannot be empty as this will crash the microcontroller due to a bug
+in LLVM which miss-optimizes empty loops to a `UDF` instruction in release mode.
+
 ``` rust
 {{#include ../../../../examples/idle.rs}}
 ```