If you have custom fields on an event, and you try to find an event with a specific value in a custom field, the api ignores that. E.g.
$result = civicrm_api3('Event', 'get', array(
'sequential' => 1,
'custom_7' => 12345,
returns all events, instead of the events with 12345 as custom value in the custom field with ID 7.