I have been seeing persistent errors of "Cannot restart clock because task does not contain unfinished clock" when auto-resuming a saved (persistent) clock.  I believe I've tracked the error down to an extraneous space in a regexp ("^[ \t]* " when it should be "^[ \t]*").

This patch fixes the issue for me (in `org-clock-in'), and it makes the same change in `org-clock-find-position'.  I've tested this briefly and it works for me; nothing about clocking-in behavior seems to break, and persistent clocks work now.

I've also wrapped `regexp-quote' around instances where org-clock-string is used to build a regexp, in case someone puts funny characters in their org-clock-string.

Hopefully, the patch is attached correctly and Thunderbird didn't mangle it.

Cheers!
Joe V.