This is not a general problem but one specific to your install or environment.
The backtrace in both cases is identical:
db:0:kdb.enter.default> bt
Tracing pid 39711 tid 100715 td 0xfffff801d0b5e740
kdb_enter() at kdb_enter+0x37/frame 0xfffffe006631f1c0
vpanic() at vpanic+0x197/frame 0xfffffe006631f210
panic() at panic+0x43/frame 0xfffffe006631f270
trap_fatal() at trap_fatal+0x391/frame 0xfffffe006631f2d0
trap_pfault() at trap_pfault+0x4f/frame 0xfffffe006631f320
trap() at trap+0x286/frame 0xfffffe006631f430
calltrap() at calltrap+0x8/frame 0xfffffe006631f430
--- trap 0xc, rip = 0xffffffff80eebdf2, rsp = 0xfffffe006631f500, rbp = 0xfffffe006631f640 ---
sysctl_dumpentry() at sysctl_dumpentry+0x1a2/frame 0xfffffe006631f640
rn_walktree() at rn_walktree+0x98/frame 0xfffffe006631f670
sysctl_rtsock() at sysctl_rtsock+0x20d/frame 0xfffffe006631f8a0
sysctl_root_handler_locked() at sysctl_root_handler_locked+0x8a/frame 0xfffffe006631f8e0
sysctl_root() at sysctl_root+0x220/frame 0xfffffe006631f960
userland_sysctl() at userland_sysctl+0x178/frame 0xfffffe006631fa10
sys___sysctl() at sys___sysctl+0x5f/frame 0xfffffe006631fac0
amd64_syscall() at amd64_syscall+0x387/frame 0xfffffe006631fbf0
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe006631fbf0
--- syscall (202, FreeBSD ELF64, sys___sysctl), rip = 0x80046ce2a, rsp = 0x7fffffffea38, rbp = 0x7fffffffea70 ---
In the past, sysctl panics have sometimes been related to BIOS/hardware quirks and are not necessarily a software problem.
Since you seem to be able to repeat this, the first step is to move it up to a 2.6.0 development snapshot and see if the problem still happens there.
Updated by 
Updated by 
Updated by