Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /path/to/wordpress3/html/wp-includes/functions.php on line 3321
You MUST turn on PHP notices while developing, otherwise you won't realize it when you're using poor code. That notice is a bit tricky to track down, but something in your plugin is using a deprecated call, which triggers the error inside of WP's functions.php. As soon as I deactivate your plugin, the notice disappears.
Most users will not trust code that throws notices.