When your opponent returns from vacation in less than 24 hours, time is added to the clock.
http://www.chess.com/forum/view/help-support/got-24hrs-added-to-my-time
Time Creation
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This is most unfair in my opinion. By all means players can go 'on vacation', and this is done automatically for paying members, but to add extra time to their clocks is adding insult to injury. If a player is expected to move once in three days then it is quite wrong to add any 'extra time' to their clock. I have since learnt that it can be done multiple times. A player could take advantage of that to prolong a lost game indefinitely - something expressly prohibited by the 'vacation' rules.
Here's a very strange thing that happened in one of my online chess games. My opponent had just over 30 minutes left to move, when he went 'on vacation'. Next within an hour or so he was back off vacation and his clock showed that he now had over 20 hours to move. How did he manage to gain the extra time?