diff --git a/examples/rust/blink/src/lib.rs b/examples/rust/blink/src/lib.rs
index 4903558f..c6529d5b 100644
--- a/examples/rust/blink/src/lib.rs
+++ b/examples/rust/blink/src/lib.rs
@@ -17,6 +17,10 @@
 //! The board must possess at least one LED and one timer. The applet indefinitely cycles through
 //! all LEDs in order with a period of 1 second and blinks them for 500 milli-seconds.
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/led.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/examples/rust/button/src/lib.rs b/examples/rust/button/src/lib.rs
index 57b9136b..243e17dc 100644
--- a/examples/rust/button/src/lib.rs
+++ b/examples/rust/button/src/lib.rs
@@ -16,6 +16,10 @@
 //!
 //! The applet prints the button state to the debug output on any button event.
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/button1.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/examples/rust/button_abort/src/lib.rs b/examples/rust/button_abort/src/lib.rs
index 14b78b89..4363184e 100644
--- a/examples/rust/button_abort/src/lib.rs
+++ b/examples/rust/button_abort/src/lib.rs
@@ -20,6 +20,10 @@
 //! If the button is released before the timer runs out, then the LED will blink until next button
 //! press.
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/timer.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/examples/rust/led/src/lib.rs b/examples/rust/led/src/lib.rs
index afeddcf1..c18f4b64 100644
--- a/examples/rust/led/src/lib.rs
+++ b/examples/rust/led/src/lib.rs
@@ -20,6 +20,10 @@
 //! is toggled. After a button is released, it becomes associated with the next LED (wrapping back
 //! to the first LED after the last LED).
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/button2.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/examples/rust/memory_game/src/lib.rs b/examples/rust/memory_game/src/lib.rs
index 1db71fbc..78762189 100644
--- a/examples/rust/memory_game/src/lib.rs
+++ b/examples/rust/memory_game/src/lib.rs
@@ -18,6 +18,10 @@
 //! serial. It essentially shows a random base32 string (the length representing the level in the
 //! game) for 3 seconds and gives 7 seconds to type it back.
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/usb.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/examples/rust/store/src/lib.rs b/examples/rust/store/src/lib.rs
index e9dab63f..9cad00e1 100644
--- a/examples/rust/store/src/lib.rs
+++ b/examples/rust/store/src/lib.rs
@@ -17,6 +17,10 @@
 //! The applet additionally relies on USB serial. It describes its own usage when connecting on the
 //! USB serial.
 
+// DO NOT EDIT MANUALLY:
+// - Edit book/src/applet/prelude/store.rs instead.
+// - Then use ./scripts/sync.sh to generate this file.
+
 #![no_std]
 wasefire::applet!();
 
diff --git a/scripts/log.sh b/scripts/log.sh
index 80a506d4..087a0dad 100644
--- a/scripts/log.sh
+++ b/scripts/log.sh
@@ -14,6 +14,8 @@
 
 x() { ( set -x; "$@"; ); }
 i() { _log '1;36' Info "$*"; }
+w() { _log '1;33' Warn "$*"; }
+t() { _log '1;33' Todo "$*"; }
 d() { _log '1;32' Done "$*"; exit 0; }
 e() { _log '1;31' Error "$*"; exit 1; }
 
diff --git a/scripts/sync.sh b/scripts/sync.sh
index 0ee7558c..0225d1ce 100755
--- a/scripts/sync.sh
+++ b/scripts/sync.sh
@@ -21,7 +21,25 @@ set -e
 cargo xtask update-apis
 
 book_example() {
-  sed '/ANCHOR/d' book/src/applet/prelude/$1.rs > examples/rust/$2/src/lib.rs
+  local src=book/src/applet/prelude/$1.rs
+  local dst=examples/rust/$2/src/lib.rs
+  # We only check that the destination is newer by more than one second, because when cloning the
+  # repository or switching branches, it may happen that the destination is slightly newer.
+  if [ $(stat -c%Y $dst) -gt $(($(stat -c%Y $src) + 1)) ]; then
+    t "Update $src instead of $dst"
+    e "$dst seems to have been manually modified"
+  fi
+  # Besides removing all anchors, we insert a warning before the #![no_std] line, which all examples
+  # should have near the beginning of the file.
+  sed '/ANCHOR/d;/^#!\[no_std\]$/{i \'"
+// DO NOT EDIT MANUALLY:\\
+// - Edit $src instead.\\
+// - Then use ./scripts/sync.sh to generate this file.\\
+
+}" $src > $dst
+  # Now that the destination has been updated, it is newer than the source. So we touch the source
+  # to preserve the invariant that the destination is never newer than the source.
+  touch $src
 }
 
 book_example led blink