Found in FreeBSD-CURRENT list posted by Robert Watson.

These warnings are generated by Witness, a run-time lock diagnostic system found in FreeBSD 5-CURRENT kernels (but removed in releases). You can read more about Witness in the WITNESS(4) man page, which talks about its capabilities. Among other things, Witness performs run-time lock order verification using a combination of hard coded lock orders, and run-time detected lock orders, and generates console warnings when lock orders are violated.

The intent of this is to detect the potential for deadlocks due to lock order violations; it's worth observing that Witness is actually slightly conservative, and so it's possible to get false positives. In the event that Witness is accurately reporting a lock order problem, it's basically saying "If you were unlucky, a deadlock would have happened here".

There are a couple of "well known" false positives, which we need to do a better job of documenting to prevent spurious reports. The non-well-known ones typically correspond to bugs in newly added locking, as lock order reversals usually get fixed pretty quickly because Witness is busy generating warnings :-).