Sometimes custom modeline chokes... Ever had custom modeline that worked 95% of time?

[Chris Seberino]

(setq default-mode-line-format (list "" 'mode-line-buffer-identification "     "
   'mode-line-system-identification "     Line %l Col %c     "
   (substring display-time-string 0 7)))


Sometimes out of the blue Emacs will tell me that the last piece
which is the time part in custom modeline above has problems.
Any reason why positions of beginning and end of last piece
would only work 95% of the time???

Chris