BUG #12856: No space left on device, but there is space!

The following bug has been logged on the website:

Bug reference:      12856
Logged by:          Adam Groszer
Email address:      agroszer@gmail.com
PostgreSQL version: 9.4.1
Operating system:   ubuntu 14.04
Description:

After our tests create a lot of DBs, around 50-100, I get this exception:

psycopg2.OperationalError: could not create file "base/24468164/24468205":
No space left on device

Weird is that 'df' shows over 60G free space on the mount and the tests
definitely won't use that much space.

After removing most of the DBs everything is back to normal, till DBs start
to pile up again.

What to check to help debugging this?

thanks,
Adam