because the two if statements fire one after the other, it will empty the mug then instantly refill it. add an else:
package coffeeDrink
{   
   function fxDTSBrick::OnActivate(%obj, %player, %client, %pos, %vec)
   {
      %n = %obj.getDatablock().getName();
      if(%n $= "mugfullData")
      {
         %obj.setDatablock(mugEmptyData);
         if(getRandom(0,20) == 13 && isObject(%player.client))
            messageClient(%player.client,'',"\c3javajavajavajavajavajavajavajavajavajava");
      } else if(%n $= "mugEmptyData")
      {
         %obj.setDatablock(mugFullData);
      }
      parent::OnActivate(%obj, %player, %client, %pos, %vec);
   }
};
activatePackage(coffeeDrink);disregard easter egg it does not exist