diff --git a/toolkit/modules/ClientID.jsm b/toolkit/modules/ClientID.jsm
index c557e220eda..019b19c7962 100644
--- a/toolkit/modules/ClientID.jsm
+++ b/toolkit/modules/ClientID.jsm
@@ -48,6 +48,11 @@ this.ClientID = Object.freeze({
    * data reporting (FHR & Telemetry). Previously exising FHR client IDs are
    * migrated to this.
    *
+   * WARNING: This functionality is duplicated for Android (see GeckoProfile.getClientId
+   * for more). There are Java tests (TestGeckoProfile) to ensure the functionality is
+   * consistent and Gecko tests to come (bug 1249156). However, THIS IS NOT FOOLPROOF.
+   * Be careful when changing this code and, in particular, the underlying file format.
+   *
    * @return {Promise<string>} The stable client ID.
    */
   getClientID: function() {