I got an interesting one here for you.
We've been trying to enter an event that has a NAME of: Paint out and Oil Painting Demo. The original submitted event has a hyphen, like so: Paint-out and Oil Painting Demo.
When I enter the event in the admin WITH the hyphen, I get the following error:
You don't have permission to access /admin/events_update.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
So I made a test event entry, successfully and went back in and edited the test event with the new info, one field at a time. Every time I entered the NAME field WITH the hyphen, it gave me the same error. When I enter it WITHOUT the hyphen, it goes through fine.
So once I got it working, I went back in and tried changing it to have the hyphen and it crashed with the same error.