PHP Warning: mysql_close(): 90 is not a valid MySQL-Link

I'm testing my website and have also made some orders. After the order confirmation (at the moment the customer has to pay) I cancel the progress.

In the order overview I have some orders that are still pending or are cancelled.

When I remove them I got two errors in the error log:

2014-05-14 15:56:07 - PHP Warning: mysql_close(): 90 is not a valid MySQL-Link resource in /home/hfori/domains/mywebshop.com/public_html/system/database/mysql.php on line 76
2014-05-14 15:56:16 - PHP Warning: mysql_close(): 90 is not a valid MySQL-Link resource in /home/hfori/domains/mywebshop.com/public_html/system/database/mysql.php on line 76

Any ideas what is causing this error and if it can do any harm?

Thanks!

Joost