The $next_title and $prev_title variables are already passed through check_plain() in forum.module, line 890 and 899, so running them through t() does it again, which causes any " characters to become "quot and print out really poorly. I recommend against using t() for the text in the URL on lines 29 and 32.
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Comment #1
michelleSee http://drupal.org/node/869006#comment-3662926
Michelle