An old client recently came to me asking to help move their site to a new host.
I've moved Drupal sites many times before (at one of my old jobs we built them on a testing server and then moved them for launch). I've never encountered this problem before.
When I first moved all the files and edited Settings.php for the imported DB I was getting a 500 Server Error. I renamed the htaccess file and suddenly the site appeared just fine. So I went to log in as the admin and boom - 404 Page Not Found. I started clicking links. Everything outside of the homepage is a 404 error. So I made a temp htaccess file and put in the small block that just handles clean URLs. Everything in the site is fine now.
Regarding the 500 error and the original htaccess, what obvious things well known to cause this error should I look out for? The file seems fine to me, so I guess I will reintegrate it line by line until I find the error. I just don't wanna end up leaving out anything crucial for Drupal to be stable or secure.