This article provides information about the support extended by the MX-series platform for the Pop without Push and Push without Pop of VLAN tags.
If you pop VLAN in an interface, you must also push some VLAN and vice versa. You also must do it for the same amount of VLANs; that is it needs to be 100% symmetric.
There are realistic scenarios, in which you might want to pop but not push or push but not pop, or pop a different amount, than the push amount.
This feature is supported on Cisco ES+ and ES20 line cards.
So, if the remote end is in such a device and is asymmetric, you must also be able to interoperate (maybe you cannot convince the remote end to change the configuration or the remote end has another device, which is hardware limited).
As of Junos 11.2R1, it is not supported; it may be added in future releases as an Enhancement Request.
Possible configuration for both scenarios:
Pop without push:
VLAN 42 comes in on the trunk port (or one with vlan-bridge encap); take it off and send it out of an access port (or one with ethernet-bridge encap). Both IFL's are in the same Bridge Domain. The configuration is as follows: