Custom fields randomly stop working

Question

For some odd reason custom fields keep randomly stop working - they're working one minute, then not the next and i do not understand why. The post, meta names and values are all still active - nothing has changed at all.

Any idea what the issue may be? The code i'm using is below and is contained within the header.php file - to re-iterate, it was working fine, then randomly stopped working - this happens pretty much every time i use custom fields.

    <div class="slide-title">
        <?php echo get_post_meta( get_the_ID(), 'slide_1_title', true ); ?>
    </div>
    <div class="slide-content">
        <?php echo get_post_meta( get_the_ID(), 'slide_1_content', true ); ?>
    </div>
    <div class="slide-headline">
        <?php echo get_post_meta( get_the_ID(), 'slide_1_headline', true ); ?>
    </div>
        <a href="#"><img src="/images/slides/slide1.jpg"></a>

Answer 1

get_the_ID() is a post tag. It is meant to be used inside a Loop. Per a comment above you are using it outside the Loop.

No, it's not inside Loop. The code is simply posted within another div, which holds slides for a slideshow.

get_the_ID like most post tags depends on a global variable called $post. If that variable is not populated, or is set to the wrong post, you get odd results. I think that is why you think this works 'randomly'.

A quirk of WordPress functioning is that the $post object is set well before the theme templates load, but only for some pages, which means for some pages some functions will work even when used incorrectly outside a Loop.

I think you probably want to be using get_queried_object to get the ID rather than depending on get_the_ID() but there are caveats with that too and the context you are using this in is still not clear.

$pobj = get_queried_object();
$id = $pobj->ID;

Related:

https://wordpress.stackexchange.com/a/98138/21376
https://wordpress.stackexchange.com/a/99545/21376
https://wordpress.stackexchange.com/a/91254/21376