Some high-security devices that don’t respond to ping triggered the following:
%ADJ-5-RESOLVE_REQ_FAIL: Adj resolve request failed for 192.168.1.1 on GigabitEthernet1/0/1
Cisco advised me that these are not an issue, and can be safely disabled with:
no ip cef optimize neighbor resolution
This apparently used to be disabled by default, but has since been enabled.