With a 4.99.22/i386 laptop, 1 July source, DIAGNOSTIC and DEBUG, I get

panic: lock error
breakpoint(ca757ea8,c,ca757ecc,c0384200,c060ef56) at netbsd:breakpoint+0x4
cpu_Debugger(c060ef56,ca757ed8,0,0,ca757ed8) at netbsd:cpu_Debugger+0xb
panic(c060e3ab,c060a514,c060a67d,c060a68f,c06ff2c0) at netbsd:panic+0xcc
lockdebug_abort(0,c06ff2c0,c06a7e6c,c060a67d,c060a68f) at netbsd:lockdebug_abort+0x6a
mutex_abort(c06ff2c0,c060a67d,c060a68f,c06fdb58,4) at netbsd:mutex_abort+0x4c
mutex_vector_exit(c06ff2c0,9,ca757f7c,c036856c,ca761a80) at netbsd:mutex_vector_exit+0x38
lwp_unlock(ca761a80,468d0a0d,5a2bcc56,c04292c4,d) at netbsd:lwp_unlock+0x6cbc7
sleepq_timeout(ca761a80,d,f4ed,0,0) at netbsd:sleepq_timeout+0x2c
softclock(0,0,ca757fec,c0701340,c0d75f80) at netbsd:softclock+0x3d6
softintr_dispatch(0,ca6cbbd4,5b379eee,7bebcc31,33b71dcc) at netbsd:softintr_dispatch+0x9a
DDB lost frame for netbsd:Xsoftclock+0x31, trying 0xca757ff4
Xsoftclock() at netbsd:Xsoftclock+0x31
--- interrupt ---
0x26b5cc08:
syncing disks... panic: lock error

Not sure what happens to cause it: I just find it in this state after
leaving idle for a while. I didn't manage a core dump...

Cheers,

Patrick