Scenario:
- older laptop Fujitsu Siemens Amilo Pi 1505
- CPU: Intel(R) Core(TM)2 CPU         T5500  @ 1.66GHz (1662.54-MHz K8-class CPU)
- 2 GB main memory
- ZFS used on 500 GB disk, geli partition
- X running but only as screensaver, no X session (machine handled remotely)
- using "shutdown -p now"

Result:
- In about 70% of cases, shutdown fails with ACPI errors as follows (this is typed in from a photo I took):

------------------------------------------------------------------------------------
Jun 30 23:17:55 gandalf syslogd: exiting on signal 15
[now switching to bold highlight font from kernel output]
Waiting (max 60 seconds) for system process 'vnlru' to stop... done
Waiting (max 60 seconds) for system process 'syncer' to stop...
Syncing disks, vnodes remaining... 6 7 7 7 7 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 done
Waiting (max 60 seconds) for system thread 'bufdaemon' to stop... done
Waiting (max 60 seconds) for system thread 'bufspacedaemon-0' to stop... done
All buffers synced.
Uptime: 15h7m15s
GEOM_ELI: Device ada0s1.eli destroyed.
GEOM_ELI: Detached ada0s1.eli on last close.
ukphy0: detached
miibus1: detached
axe0: detached
uftdi0: detached
ulpt0: detached
ACPI Error: AE_ERROR, Thread 100001 could not acquire Mutex [ACPI_MTX_Caches] (0x4) (20181003/utmutex-434)
ACPI Error: Method parse/execution failed \_PTS, AE_NO_MEMORY (20181003/psparse-677)
acpi0: AcpiEnterSleepStatePrep failed - AE_NO_MEMORY

The operating system has halted.
Please press any key to reboot.
------------------------------------------------------------------------------------

- From there, I can long press the power-off button to turn off the machine.

Expected result:
- In about 30% of cases, the machine shuts down successfully. This should happen everytime.
- The issue started around FreeBSD 11.0 if I remember correctly, anyway up to two years ago (maybe earlier).
- There definitely was a time when this always worked.

-- Martin