Be doubly-certain that the incomming CDC class driver's endpoint/pipe is flushed only once when the bank is empty.

LUFA/Drivers/USB/Class/Device/CDC.c

@@ -190,11 +190,18 @@ uint16_t CDC_Device_BytesReceived(USB_ClassInfo_CDC_Device_t* const CDCInterface
 
 	Endpoint_SelectEndpoint(CDCInterfaceInfo->Config.DataOUTEndpointNumber);
 
-	if (Endpoint_IsOUTReceived() && !(Endpoint_BytesInEndpoint()))
+	if (Endpoint_IsOUTReceived())
+	{
+		if (!(Endpoint_BytesInEndpoint()))
 		  Endpoint_ClearOUT();
 
 		return Endpoint_BytesInEndpoint();
 	}
+	else
+	{
+		return 0;
+	}
+}
 
 uint8_t CDC_Device_ReceiveByte(USB_ClassInfo_CDC_Device_t* CDCInterfaceInfo)
 {

LUFA/Drivers/USB/Class/Host/CDC.c

@@ -309,7 +309,9 @@ uint16_t CDC_Host_BytesReceived(USB_ClassInfo_CDC_Host_t* const CDCInterfaceInfo
 	Pipe_SetPipeToken(PIPE_TOKEN_IN);
 	Pipe_Unfreeze();
 
-	if (Pipe_IsINReceived() && !(Pipe_BytesInPipe()))
+	if (Pipe_IsINReceived())
+	{
+		if (!(Pipe_BytesInPipe()))
 		  Pipe_ClearIN();
 		
 		BytesInPipe = Pipe_BytesInPipe();
@@ -317,6 +319,13 @@ uint16_t CDC_Host_BytesReceived(USB_ClassInfo_CDC_Host_t* const CDCInterfaceInfo
 		
 		return BytesInPipe;
 	}
+	else
+	{
+		Pipe_Freeze();
+		
+		return 0;
+	}
+}
 
 uint8_t CDC_Host_ReceiveByte(USB_ClassInfo_CDC_Host_t* const CDCInterfaceInfo)
 {