Why are these examples of dangerous code?

Example I

1. function example_one($nid) {
2.   // Lots of code
3.   $node = node_load($nid);  
4.   // ...
5.   $user = user_load(array('uid' => $node->uid));
6. }

Example II

1. function example_two($account) {
2.   // ...
3.   $account = user_load(array('uid' => $uid));
4. }