From 5ff5682b114354577dad775dd2d9b653cf531f0f Mon Sep 17 00:00:00 2001
From: kegsay <7190048+kegsay@users.noreply.github.com>
Date: Thu, 4 Jan 2024 17:19:59 +0000
Subject: [PATCH] Update 3884-sliding-sync-e2ee.md

---
 proposals/3884-sliding-sync-e2ee.md | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/proposals/3884-sliding-sync-e2ee.md b/proposals/3884-sliding-sync-e2ee.md
index 0987bf81..32d58e18 100644
--- a/proposals/3884-sliding-sync-e2ee.md
+++ b/proposals/3884-sliding-sync-e2ee.md
@@ -47,6 +47,13 @@ _changed_ fields are returned. That is to say, if `device_one_time_keys_count` h
 requests, it will be omitted which means to use the previous value. This deviates from the current
 `/sync` implementation which always includes this field. Likewise for `device_unused_fallback_key_types`.
 
+Particular care must be taken when a fallback key is used, as this will cause the response to be:
+```json
+"device_unused_fallback_key_types": []
+```
+which is not the same as the field being omitted/null. The empty array means the key was used. Omitted
+fields means no changes.
+
 ## Potential issues
 
 It's unclear if `device_unused_fallback_key_types` and `device_one_time_keys_count` should always be