When adding a new post to a custom post type, some of the developer custom fields I had defined were throwing errors (note, I have wp_debug set to true). I traced the error and found that when adding a new post, i.e. it does not have an ID yet, property_exists($post, 'ID') coughs up the following error.
Warning: First parameter must either be an object or the name of an existing class in *domain*/wp-content/plugins/developers-custom-fields/slt-cf-init.php on line 455
I changed the property_exists function to isset($post->ID) and it seems to be solved. I don't know if this is better, worse, or a wash, but I have stopped the errors.
Edited to add: This is located in the slt-cf-init.php file.