Recently I accidentally set all posts featured image to the most recent uploaded image. Here is a snippet to fix it:

reset featured images

<?php

add_action( 'init', 'ap_reset_featured_images' );
function ap_reset_featured_images() {
  	
	if ( !is_user_logged_in() ) return;
	
	if ( isset( $_GET['reset-featured-image'] ) && true == $_GET['reset-featured-image'] ) {
	
		$allposts = get_posts( array( 'posts_per_page'  => 9999 ) );
		foreach( $allposts as $post ) : setup_postdata( $post );
			$attached_image = get_children( array( 'post_parent' => $post->ID, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'numberposts' => '1' ) );
				if ( $attached_image ) {
					$count = 0;
					foreach ( $attached_image as $attachment_id => $attachment ) {
						if ( wp_attachment_is_image( $attachment->ID ) ) $count++;
						if ( $count >= 2 ) break;
						set_post_thumbnail( $post->ID, $attachment_id );
					}
				}
		endforeach;
		
		wp_die( __( 'All post images updated.' ), __( 'Complete' ), array( 'response' => 201 ) );
	}
}