Event Max Participant Option Not Respected
As I understand it, the max registrants is for the front-end, not the back-end. Often times when someone is manually registering someone in the back-end, they may need to override settings like the max registrations. So it doesn't take that into consideration. If you need it to be one only, then the person who is doing the registering should book it via the front-end like as if they were a customer (make sure you're not logged in when you do it). That will set them as pending since they didn't pay via your online method. Then you'd edit the record to reflect that they paid.