If you commit illegal config with commit(-confirm) the config seems to be still commited.
Expected behviour would be that if commit validation fails the commit process is broken.

georg@gw-as# set zone-policy zone illegal interface eth3
[edit]
georg@gw-as# commit
[ zone-policy zone illegal interface eth3 ]
interface eth3 does not exist on system

[edit]

georg@gw-as# set zone-policy zone illegal interface eth4
[edit]
georg@gw-as# commit-confirm
commit confirm will be automatically reboot in 10 minutes unless confirmed
Proceed? [confirm][y]
[ zone-policy zone illegal interface eth4 ]
interface eth4 does not exist on system

georg@gw-as# run show configuration | match eth4

interface eth4

georg@gw-as# set zone-policy zone illegal interface eth5
[edit]
georg@gw-as# commit-confirm
Another confirm is pending
[ zone-policy zone illegal interface eth5 ]
interface eth5 does not exist on system

georg@gw-as# run show configuration | match eth5

interface eth5

[edit]

georg@gw-as# run show zone-policy zone illegal

Name: illegal

Interfaces: eth2 eth3 eth4 eth5

From Zone:

name                                    firewall                                
----                                    --------