This has bitten me in the past.

My idiom for saving a record was

$data = array('title' => 'My article');

if ($id = $this->Post->field('id', $data)) {
$this->Post->create(array('id' => $id));
}
else {

$this->Post->create();
}
$this->Post->save($data + array('body' => 'I am the elaborate discurs');

This is correct only as long as the schema for the posts table does not define any default values. However, if it does, create() will initialize the affected fields with their default values. The following save() will happily overwrite these fields with their default values if they are not included in the data array.

The correct way to write this is

$data = array('title' => 'My article');

if ($id = $this->Post->field('id', $data)) {

$this->Post->id = $id;
}
$this->Post->create(FALSE);
$this->Post->save($data + array('body' => 'I am the elaborate discurs');

The FALSE parameter to create() prevents it from initializing fields with the default values from the schema. You can read this up in "the manual":http://book.cakephp.org/view/1031/Saving-Your-Data. Search for "Creating or updating".