While in vc-annotate mode, with a git backend, if one hides the
annotations with vc-annotate-toggle-annotation-visiblity, and
tries to run vc-annotate-show-log-revision-at-line, it will display the
message "Cannot extract revision number from the current line".
Fixable by changing (move-beginning-of-line 1) to (forward-line 0) here:

(defun vc-git-annotate-extract-revision-at-line ()
  (save-excursion
    (forward-line 0)
    (when (looking-at "\\([0-9a-f^][0-9a-f]+\\) \\(\\([^(]+\\) \\)?")
      (let ((revision (match-string-no-properties 1)))
(if (match-beginning 2)
    (let ((fname (match-string-no-properties 3)))
      ;; Remove trailing whitespace from the file name.
      (when (string-match " +\\'" fname)
(setq fname (substring fname 0 (match-beginning 0))))
      (cons revision
    (expand-file-name fname (vc-git-root default-directory))))
  revision)))))

Evgeni