The RadHtmlPlaceholder control gets misplaced when displayed inside a RadWindow in a zoomed browser. The issue can be reproduced in the HtmlPlaceholder WindowIntegration demo example.

The placeholder disappears if it's in a RadPane and you close another RadPane

When the user navigates in a page displayed in the placeholder and then it is removed and added in the visual tree (when in RadDocking for example) the original SourceUrl is displayed while ideally this should be the page that the user has navigated to.
Update: You can fix this by setting the HTMLPlaceholder's KeepContentInMemory property to True.

When displayed inside a RadWindow, if the window is moved outside the browser, the placeholder gets cut off.
This issue also appears if initally the left side of RadWindow is positioned outside the browser.

The control is misplaced if it's inside a GridRow/Column and there's the browser zoom is different than 100%.

The fix will be available in the Q1 2015 Official Release.