diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -458,6 +458,10 @@
   }
 
   public function saveCache() {
+    if (PhabricatorEnv::isReadOnly()) {
+      return false;
+    }
+
     if ($this->highlightErrors) {
       return false;
     }
diff --git a/src/applications/people/storage/PhabricatorUser.php b/src/applications/people/storage/PhabricatorUser.php
--- a/src/applications/people/storage/PhabricatorUser.php
+++ b/src/applications/people/storage/PhabricatorUser.php
@@ -968,6 +968,10 @@
    * @task availability
    */
   public function writeAvailabilityCache(array $availability, $ttl) {
+    if (PhabricatorEnv::isReadOnly()) {
+      return $this;
+    }
+
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
     queryfx(
       $this->establishConnection('w'),