For example, conf-sanity 62 causes error message:

VFS: Busy inodes after unmount of loop0. Self-destruct in 5 seconds. Have a nice day...

Similar message could be seen after each test. So we need extra check in test cleanup.

There were several issues with missed iput - for example LU-5729. Provided patch will help to find similar problems in future.
"Busy inodes" is just one of error messages that tells about internal kernel problems.
I hope list of error messages in check_dmesg_for_errors will be supplemented by developers.