diff --git a/src/libguac/client.c b/src/libguac/client.c
index 21906077e..43d61106e 100644
--- a/src/libguac/client.c
+++ b/src/libguac/client.c
@@ -186,8 +186,25 @@ static void guac_client_promote_pending_users(union sigval data) {
     guac_acquire_write_lock(&(client->__pending_users_lock));
 
     /* Run the pending join handler, if one is defined */
-    if (client->join_pending_handler)
-        client->join_pending_handler(client);
+    if (client->join_pending_handler) {
+
+        /* If an error occurs in the pending handler */
+        if(client->join_pending_handler(client)) {
+
+            guac_release_lock(&(client->__pending_users_lock));
+
+            /* Mark the handler as not running */
+            pthread_mutex_lock(&(client->__pending_users_timer_mutex));
+            client->__pending_users_timer_state = GUAC_CLIENT_PENDING_TIMER_REGISTERED;
+            pthread_mutex_unlock(&(client->__pending_users_timer_mutex));
+
+            /* Log a warning and abort the promotion of the pending users */
+            guac_client_log(client, GUAC_LOG_WARNING,
+                    "join_pending_handler did not successfully complete;"
+                    " any pending users have not been promoted.\n");
+            return;
+        }
+    }
 
     /* The first pending user in the list, if any */
     guac_user* first_user = client->__pending_users;
diff --git a/src/libguac/guacamole/client-fntypes.h b/src/libguac/guacamole/client-fntypes.h
index 26cddb6f8..367a6f1e2 100644
--- a/src/libguac/guacamole/client-fntypes.h
+++ b/src/libguac/guacamole/client-fntypes.h
@@ -57,8 +57,12 @@ typedef int guac_client_free_handler(guac_client* client);
  *
  * @param client
  *     The client whose handler was invoked.
+ *
+ * @return
+ *     Zero if the pending handler ran successfuly, or a non-zero value if an
+ *     error occured.
  */
-typedef void guac_client_join_pending_handler(guac_client* client);
+typedef int guac_client_join_pending_handler(guac_client* client);
 
 /**
  * Handler for logging messages related to a given guac_client instance.
diff --git a/src/libguac/guacamole/client.h b/src/libguac/guacamole/client.h
index c98c43b09..4bfdc0704 100644
--- a/src/libguac/guacamole/client.h
+++ b/src/libguac/guacamole/client.h
@@ -255,7 +255,7 @@ struct guac_client {
      *
      * Example:
      * @code
-     *     void join_pending_handler(guac_client* client);
+     *     int join_pending_handler(guac_client* client);
      *
      *     int guac_client_init(guac_client* client) {
      *         client->join_pending_handler = join_pending_handler;
diff --git a/src/protocols/kubernetes/client.c b/src/protocols/kubernetes/client.c
index b3a44e110..f90358fbc 100644
--- a/src/protocols/kubernetes/client.c
+++ b/src/protocols/kubernetes/client.c
@@ -85,8 +85,11 @@ static void guac_kubernetes_log(int level, const char* line) {
  * @param client
  *     The client whose pending users are about to be promoted to full users,
  *     and therefore need their connection state synchronized.
+ *
+ * @return
+ *     Always zero.
  */
-static void guac_kubernetes_join_pending_handler(guac_client* client) {
+static int guac_kubernetes_join_pending_handler(guac_client* client) {
 
     guac_kubernetes_client* kubernetes_client =
         (guac_kubernetes_client*) client->data;
@@ -97,6 +100,8 @@ static void guac_kubernetes_join_pending_handler(guac_client* client) {
     guac_kubernetes_send_current_argv_batch(client, broadcast_socket);
     guac_socket_flush(broadcast_socket);
 
+    return 0;
+
 }
 
 int guac_client_init(guac_client* client) {
diff --git a/src/protocols/rdp/client.c b/src/protocols/rdp/client.c
index 0228b7e39..5130f1944 100644
--- a/src/protocols/rdp/client.c
+++ b/src/protocols/rdp/client.c
@@ -107,8 +107,11 @@ static void* guac_rdp_sync_pending_user_audio(guac_user* user, void* data) {
  *
  * @param client
  *     The client whose pending users are about to be promoted.
+ *
+ * @return
+ *     Always zero.
  */
-static void guac_rdp_join_pending_handler(guac_client* client) {
+static int guac_rdp_join_pending_handler(guac_client* client) {
 
     guac_rdp_client* rdp_client = (guac_rdp_client*) client->data;
     guac_socket* broadcast_socket = client->pending_socket;
@@ -126,6 +129,8 @@ static void guac_rdp_join_pending_handler(guac_client* client) {
 
     guac_socket_flush(broadcast_socket);
 
+    return 0;
+
 }
 
 int guac_client_init(guac_client* client, int argc, char** argv) {
diff --git a/src/protocols/ssh/client.c b/src/protocols/ssh/client.c
index c813808b8..74a62ba07 100644
--- a/src/protocols/ssh/client.c
+++ b/src/protocols/ssh/client.c
@@ -43,8 +43,11 @@
  *
  * @param client
  *     The client whose pending users are about to be promoted.
+ *
+ * @return
+ *     Always zero.
  */
-static void guac_ssh_join_pending_handler(guac_client* client) {
+static int guac_ssh_join_pending_handler(guac_client* client) {
 
     guac_ssh_client* ssh_client = (guac_ssh_client*) client->data;
 
@@ -54,6 +57,8 @@ static void guac_ssh_join_pending_handler(guac_client* client) {
     guac_ssh_send_current_argv_batch(client, broadcast_socket);
     guac_socket_flush(broadcast_socket);
 
+    return 0;
+
 }
 
 int guac_client_init(guac_client* client) {
diff --git a/src/protocols/telnet/client.c b/src/protocols/telnet/client.c
index 6f2546324..3b4184023 100644
--- a/src/protocols/telnet/client.c
+++ b/src/protocols/telnet/client.c
@@ -42,8 +42,11 @@
  *
  * @param client
  *     The client whose pending users are about to be promoted.
+ *
+ * @return
+ *     Always zero.
  */
-static void guac_telnet_join_pending_handler(guac_client* client) {
+static int guac_telnet_join_pending_handler(guac_client* client) {
 
     guac_telnet_client* telnet_client = (guac_telnet_client*) client->data;
 
@@ -53,6 +56,8 @@ static void guac_telnet_join_pending_handler(guac_client* client) {
     guac_telnet_send_current_argv_batch(client, broadcast_socket);
     guac_socket_flush(broadcast_socket);
 
+    return 0;
+
 }
 
 int guac_client_init(guac_client* client) {
diff --git a/src/protocols/vnc/client.c b/src/protocols/vnc/client.c
index 9d7aa099d..47cafa721 100644
--- a/src/protocols/vnc/client.c
+++ b/src/protocols/vnc/client.c
@@ -70,8 +70,11 @@ static void* guac_vnc_sync_pending_user_audio(guac_user* user, void* data) {
  *
  * @param client
  *     The client whose pending users are about to be promoted.
+ *
+ * @return
+ *     Always zero.
  */
-static void guac_vnc_join_pending_handler(guac_client* client) {
+static int guac_vnc_join_pending_handler(guac_client* client) {
 
     guac_vnc_client* vnc_client = (guac_vnc_client*) client->data;
     guac_socket* broadcast_socket = client->pending_socket;
@@ -87,6 +90,8 @@ static void guac_vnc_join_pending_handler(guac_client* client) {
     guac_common_display_dup(vnc_client->display, client, broadcast_socket);
     guac_socket_flush(broadcast_socket);
 
+    return 0;
+
 }
 
 int guac_client_init(guac_client* client) {