Many posters have problems debugging their RewriteRule and RewriteCond statements within their .htaccess
files. Most of these are using a shar
One from a couple of hours that I wasted:
If you've applied all these tips and are only going on 500 errors because you don't have access to the server error log, maybe the problem isn't in the .htaccess but in the files it redirects to.
After I had fixed my .htaccess-problem I spent two more hours trying to fix it some more, even though I simply had forgotten about some permissions.