There's a bug in the BigTreeCMS class file (these files really should have revision numbers in them), line 1061, which currently reads
sqlquery("UPDATE bigtree_404s SET requests = (requests + 1) WHERE = '".$f["id"]."'");
but it's missing the object of the WHERE clause. It should read
sqlquery("UPDATE bigtree_404s SET requests = (requests + 1) WHERE id = '".$f["id"]."'");
That should fix the broken counter.
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