>
> Evaluating next sexp results in error:
>
>   (with-temp-buffer
>     (let ((fill-column 66))
>       (insert "                                   :action (lambda (docid msg target)")
>       (fill-region (point-min) (point-max) 'left t)
>       (buffer-string)))
>  

I cannot reproduce this on recent revision of the trunk (8c589c2583), it 
produces the expected result: the inserted string is displayed.