So you was thinking that site backup has no Interruption to the running (live) system ? Yes it is. I ran in to an issue where an Granular backup was executed through SharePoint Management shell while users were accessing the portal in SharePoint 2010 production farm.
Backup was terminated due to lack of space in destination drive and users are prompted with "Error: Access Denied Massage" which was in an extremely critical peak hour.
There were multiple set of backup jobs running parallel in SharePoint Shell and few of them were unable to complete due to lack of space. The person who was handling this closed all the SharePoint Shells and sites suddenly prompted this error to all users. Sites which are successfully backed up had no issues.
Checked the Content DBs of particular Web Applications and they looks green as Database Read-only mode is "No".
When Looking out for a possible reason, the "Lock" word came up to my mind and checked the "Quotas and Locks" in Central Administration (Application Management –> Site Collections –> Configure Quotas and Locks)
And Here we Go !. It was in the Read only Mode. Changing the status to the "Not Locked" Mode bought everything back to normal.
It has put the site in to maintenance mode during the backup and since it was not properly completed, the status yet remains in Read-Only mode. So a good point to think before you execute any backups in SharePoint. Plan for a drive with enough space and off-peak hour.
This is the command line resolution for the same
stsadm -o setsitelock -url http://sitename -lock readonly
stsadm -o setsitelock -url http://sitename -lock none
Here are some good facts in terms of Backup and Restore Planning in SharePoint From Microsoft – https://technet.microsoft.com/en-us/library/gg266384.aspx