In Drupal 6, if you called
node_load then what you'd be returned would be your own copy of the node to change and modify as you pleased, but in Drupal 7 this was changed. By default, what you actually get back from a
node_load is essentially a reference to a global singleton for that node. This has the interesting side effect of meaning that if you change anything in the node object, you are probably changing everyone else's copies of that node too.