Warning: A rendering was not used on the page. The placeholder "xxxxxx" was not found

Question

There is a Desktop Layout (Desktop.aspx) with several controls (Figure 1 and Figure 3). One of the controls called "Two Column Big Left" (TwoColumnBigLeft.ascx) has three placeholders (Inner Content Top, Left, Right). Then, there are six controls added to the three placeholders on the "Two Column Big Left" control.

Page Title (Inner Content Top)
Generic Body Content (Left)
Video Carousel (Left)
Capability Accordian (Left)
Generic Callout (Right)
Generic Callout (Right)

I am getting warnings in Debug mode (Figure 2) and caching is not working correctly (it is supposed to vary by data on the "Playbook Playlist" control, but it is cached permanently).

Is this considered a correct way to configure the Presentation Details? What do I need to do to get rid of the warnings? Do the placeholders all need to be on Desktop.aspx? Or, maybe the TwoColumnBigLeft.ascx page needs to be included on the Desktop.aspx page directly in the code (not through Sitecore presentation details)? Should I create a second version of the Desktop.aspx page to use for this page?

Other than the caching problem, the page looks fine in the web browser.

Figure 1

Figure 2

Figure 3

Answer 1

There's an entry for this in the Sitecore presentation component troubleshooting guide

It's this section:

If layout details indicate that the layout engine should bind a control to a placeholder, but the layout engine cannot locate a placeholder with that key in the layouts and sublayouts already processed, the layout engine does not create an error visible to the browser. Instead, the layout engine writes warning messages such as the following to the page trace visible in the Sitecore browser-based debugger:

Warning

A rendering was not used on the page

The placeholder was not found

If you locate any of these warning messages in the page trace, confirm that the sublayouts and renderings in layout details for the item specify placeholder keys that exist in the layout and sublayouts specified. Ensure that placeholder settings list controls in component nesting order. For example, a sublayout containing a placeholder should appear before the controls that bind to that placeholder. Ensure that fully qualified placeholder keys reflect the correct placeholder nesting order.

It sounds like this is a common issue - if you look in your placeholder settings, check that you have the correct placeholder controls and that they are in nested order (so make sure your 3 inner placeholders are below their parent placeholder).

If that isn't what's wrong - you could also try using fully qualified placeholder names (e.g. instead of "Right" use "/Content/Right").