Just checked, Linux usb driver decided to lose a disk during a 'stress-test' over unpacking linux source instead of triggering an assertion in 2.5 (and to irreparably damage its ext4 as well), NetBSD 7.0 reboot action hangs on USB_RESET and NetBSD 5.1 triggers second of mentioned asserts. Backend itself is a regular USB2-SATA adapter with Intel S3500 SSD, hence it should not trigger first timing-related assertion at all.
usb-msd: Data out 16384/16384 usb-msd: Command complete 0 tag 0x9f5 usb-msd: Command status 0 tag 0x9f5, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9f6 flags 00000000 len 10 data 16384 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x00 0x7f 0x00 0x00 0x20 0x00 - to-dev len=16384 usb-msd: Data out 16384/16384 usb-msd: Command complete 0 tag 0x9f6 usb-msd: Command status 0 tag 0x9f6, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9f7 flags 00000000 len 10 data 16384 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x00 0x9f 0x00 0x00 0x20 0x00 - to-dev len=16384 usb-msd: Data out 16384/16384 usb-msd: Command complete 0 tag 0x9f7 usb-msd: Command status 0 tag 0x9f7, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9f8 flags 00000000 len 10 data 16384 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x00 0xbf 0x00 0x00 0x20 0x00 - to-dev len=16384 usb-msd: Data out 16384/16384 usb-msd: Command complete 0 tag 0x9f8 usb-msd: Command status 0 tag 0x9f8, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9f9 flags 00000000 len 10 data 4096 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x02 0xbb 0x5f 0x00 0x00 0x08 0x00 - to-dev len=4096 usb-msd: Data out 4096/4096 usb-msd: Command complete 0 tag 0x9f9 usb-msd: Command status 0 tag 0x9f9, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9fa flags 00000000 len 10 data 2048 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x30 0x4c 0xbb 0x00 0x00 0x04 0x00 - to-dev len=2048 usb-msd: Data out 2048/2048 usb-msd: Command complete 0 tag 0x9fa usb-msd: Command status 0 tag 0x9fa, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9fb flags 00000000 len 10 data 2048 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x00 0x4f 0x00 0x00 0x04 0x00 - to-dev len=2048 usb-msd: Data out 2048/2048 usb-msd: Command complete 0 tag 0x9fb usb-msd: Command status 0 tag 0x9fb, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9fc flags 00000000 len 10 data 2048 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x17 0x1f 0x00 0x00 0x04 0x00 - to-dev len=2048 usb-msd: Data out 2048/2048 usb-msd: Command complete 0 tag 0x9fc usb-msd: Command status 0 tag 0x9fc, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9fd flags 00000000 len 10 data 2048 [usb-disk0.0 id=0] WRITE_10 0x00 0x00 0x00 0x00 0x4f 0x00 0x00 0x04 0x00 - to-dev len=2048 usb-msd: Data out 2048/2048 usb-msd: Command complete 0 tag 0x9fd usb-msd: Command status 0 tag 0x9fd, len 13 usb-msd: Command on LUN 0 usb-msd: Command tag 0x9fe flags 00000000 len 10 data 0 [usb-disk0.0 id=0] SYNCHRONIZE_CACHE 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 - none usb-msd: Deferring packet 0x7f335af66630 [wait status] qemu-system-x86_64: hw/usb/core.c:491: usb_cancel_packet: Assertion `usb_packet_is_inflight(p)' failed. 2016-01-15 17:53:11.443+0000: shutting down
