I botched the upgrade to v3.4.0 on one of my BRs, getting the kernel module and the user-space utility out of sync. Therefore Jool ended up in an unconfigured state. The unfortunate thing is that the start-up scripts incorrectly believed that Jool was initialised just fine, and therefore proceeded to start up BIRD to advertise routes to the various IP pools to the rest of the network. This caused blackholing of traffic.
The problem would not have occurred if the user-space app had exited unsuccessfully when encountering any error.
$ /usr/bin/jool_siit -e
Error: (Error code: 22)
$ echo $?
$ dmesg | tail -n 1
[ 461.694810] SIIT Jool ERROR (validate_version): Version mismatch. The kernel module is 184.108.40.206, but the userspace application is 220.127.116.11. Please update Jool's userspace application.
The error printing function was clearing the error code.