Actual Templates

costumes (approximated by example A)

 {embed="site/.part-header" title="Costumes"}
 
 <h1>Costumes</h1>
 
 <div class="category-nav">
    <ul>
        <li class="previous-concept"><a href="" class="disabled">Previous Concept</a></li>
        <li class="back-to-grid"><a href="" class="disabled">Back to Grid</a></li>
        
        {exp:channel:categories channel="costumes" style="linear" parent_only="yes"}
            {if count == "1"}
            {exp:channel:entries category="{category_id}" orderby="title" sort="asc" limit="1"}
            <li class="next-concept"><a href="{site_url}costume-detail/{entry_id}/">Next Concept</a></li>
            {/exp:channel:entries}
            {/if}
        {/exp:channel:categories}
    </ul>
    <br class="clear" />
 </div>
    
 <div class="costume-grid">
 {exp:query sql="SELECT cat_name as parent_category_name, cat_id as parent_category_id 
                FROM exp_categories 
                WHERE group_id = 4 
                AND parent_id = 0 
                ORDER BY cat_order ASC"}
    <div class="costume-grid-row{if count == total_results} last{/if}">
        <div class="gridbox">
            <p class="costumecategory">{parent_category_name}</p>
        </div>
 
        {embed="site/.part-category-item" category_id="{parent_category_id}"  base_url="{site_url}costume-detail/"}
 
        <br class="clear" />
    </div>
 {/exp:query}
 </div>
 
 <div class="intros">
    <div class="graybox">
        {exp:channel:entries channel="static" entry_id="11" limit="1"}
            <h2>{title}</h2>
            {content-1}
        {/exp:channel:entries}
 
        <div class="right">
            {exp:channel:entries channel="costumes" dynamic="no" category="60" limit="1" sort="asc"}
                <a id="explore" class="text-button" href="{site_url}costume-detail/{url_title}/">Explore</a>
            {/exp:channel:entries}
        </div>
    </div>
 </div>
 
 {embed="site/.part-footer"}

.part-category-item

 {exp:query sql="SELECT c.cat_id, c.cat_url_title, c.cat_name 
                FROM exp_channel_titles t 
                LEFT JOIN exp_channel_data d ON t.entry_id = d.entry_id 
                LEFT JOIN exp_category_posts cp ON t.entry_id = cp.entry_id
                LEFT JOIN exp_categories c ON cp.cat_id = c.cat_id
                WHERE c.parent_id = '{embed:category_id}'
                ORDER BY c.cat_order ASC"}
    {exp:channel:entries category="{cat_id}" orderby="title" sort="asc" dynamic="no"}
        <div class="gridbox" style="overflow: visible;">
            {categories show="not {embed:category_id}" limit="1"}
                <table cellspacing="0" cellpadding="0" style="overflow: visible;">
                    <tr style="overflow: visible;">
                        <td style="overflow: visible;">
                            <a href="{embed:base_url}{url_title}/" >
                                <img src="{category_image}" style="margin: 0; padding: 0;" title="{title}" />
                            </a>
                        </td>
                    </tr>
                </table>
                <p><a href="{embed:base_url}{url_title}/">{category_name}</a></p>
            {/categories}
        </div>
    {/exp:channel:entries}
 {/exp:query}

.part-category-item-test (identical to previous, except for filename)

 {exp:query sql="SELECT c.cat_id, c.cat_url_title, c.cat_name 
                FROM exp_channel_titles t 
                LEFT JOIN exp_channel_data d ON t.entry_id = d.entry_id 
                LEFT JOIN exp_category_posts cp ON t.entry_id = cp.entry_id
                LEFT JOIN exp_categories c ON cp.cat_id = c.cat_id
                WHERE c.parent_id = '{embed:category_id}'
                ORDER BY c.cat_order ASC"}
    {exp:channel:entries category="{cat_id}" orderby="title" sort="asc" dynamic="no"}
        <div class="gridbox" style="overflow: visible;">
            {categories show="not {embed:category_id}" limit="1"}
                <table cellspacing="0" cellpadding="0" style="overflow: visible;">
                    <tr style="overflow: visible;">
                        <td style="overflow: visible;">
                            <a href="{embed:base_url}{url_title}/" >
                                <img src="{category_image}" style="margin: 0; padding: 0;" title="{title}" />
                            </a>
                        </td>
                    </tr>
                </table>
                <p><a href="{embed:base_url}{url_title}/">{category_name}</a></p>
            {/categories}
        </div>
    {/exp:channel:entries}
 {/exp:query}

Update: I've now added these templates just in case there's something else to be gleaned, but deleting the original .part-category-item and renaming the (identical) .part-category-item-test worked. At this point I assume it's impossible to narrow down to the answer without also sharing the database, asking the person who migrated the site exactly what he did, etc. I'll chalk it up to some kind of storage or encoding issue, but that's still just a guess.

I can't create and accept my own answer because I don't think this constitutes one – just a fix. I'm fine with Stack Overflow but end up in messy situations like this on the EE Stack Exchange. Open to suggestions!

When I use an embed tag within either an exp:query or exp:channel:categories loop, the outcome is the same: the embedded template erroneously receives the same variable over and over.

Pretend we have three categories, with IDs of 1, 2, and 3.

Example Embed Template (site/template)

 The parent category ID is {embed:parent_category_id}.<br/>

Example A

 {exp:query sql="SELECT cat_id as category_id FROM exp_categories LIMIT 3"}
      {category_id} / {embed="site/template" parent_category_id="{category_id}"}
 {/exp:query}

outputs...

 1 / The parent category ID is 1.
 2 / The parent category ID is 1.
 3 / The parent category ID is 1.

Example B

 {exp:channel:categories style="linear" parent_only="yes"}
      {embed="site/template" parent_category_id="{category_id}"}
 {/exp:query}

outputs the same thing...

 1 / The parent category ID is 1.
 2 / The parent category ID is 1.
 3 / The parent category ID is 1.

Things I've tried (unsuccessfully):

• changing up variable names
• liberally applying parse="inward" in various combinations
• using a conditional like {if count == 1}foo{if:else}bar{/if} to see if I can get the printed value of {embed:parent_category_id} to change.

I've used embeds within loops many, many times without experiencing this issue, and it's driving me bonkers. There's no caching or additional fanciness in play that I can think of.

EE 2.7.2.

When I use an embed tag within either an exp:query or exp:channel:categories loop, the outcome is the same: the embedded template erroneously receives the same variable over and over.

Pretend we have three categories, with IDs of 1, 2, and 3.

Example Embed Template (site/template)

 The parent category ID is {embed:parent_category_id}.<br/>

Example A

 {exp:query sql="SELECT cat_id as category_id FROM exp_categories LIMIT 3"}
      {category_id} / {embed="site/template" parent_category_id="{category_id}"}
 {/exp:query}

outputs...

 1 / The parent category ID is 1.
 2 / The parent category ID is 1.
 3 / The parent category ID is 1.

Example B

 {exp:channel:categories style="linear" parent_only="yes"}
      {category_id} / {embed="site/template" parent_category_id="{category_id}"}
 {/exp:channel:categories}

outputs the same thing...

 1 / The parent category ID is 1.
 2 / The parent category ID is 1.
 3 / The parent category ID is 1.

Things I've tried (unsuccessfully):

• changing up variable names
• liberally applying parse="inward" in various combinations
• using a conditional like {if count == 1}foo{if:else}bar{/if} to see if I can get the printed value of {embed:parent_category_id} to change.

I've used embeds within loops many, many times without experiencing this issue, and it's driving me bonkers. There's no caching or additional fanciness in play that I can think of.

EE 2.7.2.