When developing plugins or themes for WordPress, please, please, please, add this bit of code to your test site’s
define('WP_DEBUG', true); # Debugging
If your test site is public and you only want the debug information to show up for yourself, restrict it to your IP address:
if ($_SERVER['REMOTE_ADDR'] == 'xx.x.xx.xxx') define('WP_DEBUG', true);
There's no reason to release a plugin or theme into the wild that's laden with uninitialized variables, deprecated functions, undefined indexes, and so on. Turning debugging on will force you to either ignore the warning messages or will encourage you to clean up your code.
Now you know, and knowing is half the battle. The other half is outclassing the competition. :)