Suppose you want to complete an org-capture template with this: “%^{Title}”.
When you call this capture, the minibuffer allows you to select the title (in the minibuffer). But, if you want to capture "from" this minibuffer, it crashes org-capture with the error (just after the choice of the second template) : 

org-capture: Capture template 'u': Can't expand minibuffer to full frame.

Concrete use case: capturing an org-roam node (and complete the title), trying to insert a node that doesn't yet exist (and therefore reopening org-capture) in the title of the first capture process.