>
> How to reproduce from emacs -Q:
>
>  (setq enable-recursive-minibuffers t)
>  (setq icomplete-show-matches-on-no-input t)
>  M-x icomplete-mode RET
>  C-x C-f
>    [Icomplete now exhibits some filenames in the minibuffer]
>  M-:
>    [Icomplete shouldn’t exhibit anything here but it still
>     exhibits the filenames from the previous step and even
>     allows you to complete them with C-j]
>
> You can replace C-x C-f with other similar commands, e.g., C-x b or M-x. 
> I’m not sure if this bug comes from how Icomplete configures itself or 
> the implementation of recursive minibuffers.  It appears that 
> icomplete-simple-completing-p returns t when the M-: is happening in a 
> recursive minibuffer, even though it should return nil so that Icomplete 
> doesn’t configure itself.
>
> If the M-: is not in a recursive minibuffer, everything is OK since 
> icomplete-simple-completing-p returns nil.
>

Indeed; patch attached.  Ideally this should be done inside 
icomplete-minibuffer-setup, but if we did this (by checking 
minibuffer-completing-symbol), it would prevent icomplete-mode from being 
active in the opposite situation: M-: followed by C-x C-f.