The syntax for mod_rewrite rules can be tricky and hard to get just right, especially as they get more complex. The rewrite log can help debug RewriteRules. However, as soon as you debug your rules, it's very important to turn it off.
Because a single rewrite rule can produce several lines of output, the rewrite engine obtains a lock on the rewrite log for each access. This means your site can only process one access at a time, which is a horrible performance penalty.
Worse than that, we have seen Apache occasionally lose this lock if an in-progress request dies during rewrite processing (for example due to a closed connection), which will cause all requests for your site to stall indefinitely until the server won't accept any more.
For these reasons, you absolutely should not run the rewrite log on a production site, or for any longer than absolutely necessary to debug your rewrite rules.