Once every six weeks or so, the Mortal Kombat 1 servers are taken offline for maintenance. During this time, players may find that they're unable to make use of their DLC characters, as the game seems to have no way of verifying that they hold the correct licenses. Normally, maintenance only lasts for a few hours, so this isn't a massive issue, but there have been times when the MK1 servers have been down for much longer periods.