One way I get locked out while developing is by changing the permissions on any of the directories or files associated with the keys used by ssh. This happens to me when I untar files or run some installation script while initializing the machine that touches the ec2-user account.
1> ls -ld $HOME $HOME/.ssh
drwx------ 15 ec2-user ec2-user 4096 Aug 1 13:15 /home/ec2-user/
drwx------ 2 ec2-user ec2-user 4096 Jun 10 11:18 /home/ec2-user/.ssh/
2> ls -l $HOME/.ssh/authorized_keys
-rw------- 1 ec2-user ec2-user 385 Jun 10 11:18 /home/ec2-user/.ssh/authorized_keys
To protect myself against my own mistakes, I create a new user and copy the ec2-user key over. That account is used as a backup and not part of any script. Once I tested everything, I remove the account.