diff --git a/CHANGELOG b/CHANGELOG
index d9f8d449..e2f3b686 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -499,6 +499,10 @@ HISTORY
 
   ++ Bug fixes:
 
+  2007-11-26 Simon Goldschmidt
+  * tcp_in.c: Fixed bug #21582: pcb->acked accounting can be wrong when ACKs
+    arrive out-of-order
+
   2007-11-21 Simon Goldschmidt
   * tcp.h, tcp_out.c, api_msg.c: Fixed bug #20287: tcp_output_nagle sends too early
     Fixed the nagle algorithm; nagle now also works for all raw API applications
diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index fac1f8cf..1fcb10a6 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -833,6 +833,9 @@ tcp_receive(struct tcp_pcb *pcb)
         pcb->rtime = 0;
 
       pcb->polltmr = 0;
+    } else {
+      /* Fix bug bug #21582: out of sequence ACK, didn't really ack anything */
+      pcb->acked = 0;
     }
 
     /* We go through the ->unsent list to see if any of the segments