This is easily reproducible and have triggered it on two 11.1-RELEASE boxes.
ifconfig stf create --> kernel panic.
Adding a number to stf, eg stf0 or stf1 the interface is created as expected. This is a bug that should be fixed, but probably doesn't affect majority of users.
Also tested on head r321403 and appears to be fixed.
Do you have pf loaded? If so, this is a duplicate of #218895.
pf gets confused and upset if there are ifgroups with the same name as an ifnet.
That is the issue, thanks.
*** This bug has been marked as a duplicate of bug 218895 ***