As of Junos version 10.1 there is a change to the way MPLS label stacking is handled.
After upgrading a router from a Junos build older than Junos 10.1 to 10.1 (or higher), messages similar to the following are seen in the syslog messages:
RPD_MPLS_INTF_MAX_LABELS_ERROR: Maximum labels exceeded - failed to stack additional label
to nexthop on interface so-0/1/0.0
Having not seen these messages previously, seeing them in the logs now may cause undue alarm.
Prior to Junos 10.1, the maximum number of MPLS labels that could be stacked on a packet for an M-series router was 3 (5 on T-series and MX-series). The code to stack additional labels did not exist in the MPLS code, and if the configuration attempted to do so the label would not be written; there would be no error message
The ability to configure the maximum number of MPLS labels was introduced in Junos 10.1. You can now configure up to 5 (default is still 3) as a maximum with the command “set interfaces <interface><logical unit> family mpls maximum-labels”
Additionally, if a router attempts to add more labels, it logs the previously mentioned error message.