FreeBSD 15 recently gained a nice feature called "exterr”, which allows syscalls to return an error message string in addition to an errno value.
that's nice for lots of things, but particularly for things like if_bridge that have a lot of error cases that return EINVAL and it's not really clear what's going on.
old behaviour:
# ifconfig bridge1 untagged epair2a 20
ifconfig: BRDGSIFPVID 20: Invalid argument
new behaviour:
# ifconfig bridge1 untagged epair2a 20
ifconfig: BRDGSIFPVID 20: Invalid argument (extended error VLAN filtering not enabled)
this only works for syscalls which have specifically added support for it, so it'll take a while for everything to get the new errors.