ATA panic: Duplicate free of item 0xc9eca088 from zone 0xc1022600(g_bio)

One of the i386 package machines has panicked with the following:

ad0: TIMEOUT - WRITE_DMA retrying (2 retries left) LBA=36328872
ad0: TIMEOUT - WRITE_DMA retrying (2 retries left) LBA=37993024
usb0: 1 scheduling overruns
Slab at 0xc9ecafc8, freei 1 = 29.
panic: Duplicate free of item 0xc9eca088 from zone 0xc1022600(g_bio)

at line 271 in file ../../../vm/uma_dbg.c
Debugger("panic")
Stopped at      Debugger+0x54:  xchgl   %ebx,in_Debugger.0

Debugger(c076a7f5,c07d5e00,10f,c0782b88,100) at Debugger+0x54
__panic(c0782b88,10f,c0782c9e,c9eca088,c1022600) at __panic+0xf5
uma_dbg_free(c1022600,c9ecafc8,c9eca088,74f,17d0bac) at uma_dbg_free+0x151
uma_zfree_internal(c1022600,c9eca088,0,0,0,0,c07d09e0) at uma_zfree_internal+0x13f
uma_zfree_arg(c1022600,c9eca088,0,df4f2c64,c053bc3c) at uma_zfree_arg+0x3c2
g_destroy_bio(c9eca088,0,c07658f9,bc,c9eca088) at g_destroy_bio+0x22
g_disk_done(c9eca088,0,c0771069,bb8,c63fa4ec) at g_disk_done+0x7c
biodone(c9eca088,0,c61a625c,0,df4f2cc8) at biodone+0x80
ad_done(c63fa4ec,c63fa4ec,0,c0769b9d,116) at ad_done+0x34
ata_completed(c63fa4ec,0,c076d779,cc,c61a625c) at ata_completed+0x436
taskqueue_run(c61a6240,0,c076d779,f5,0) at taskqueue_run+0x9c
taskqueue_thread_loop(0,df4f2d48,c07681f3,328,0) at taskqueue_thread_loop+0xb8
fork_exit(c059c270,0,df4f2d48) at fork_exit+0xd5
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xdf4f2d7c, ebp = 0 ---