I just installed OpenBSD 6.1 and set up a CUPS server with a usb printer, and I'm experiencing kernel panics that seem usb related.
These kernel panics actually occured also on the old version of OpenBSD I upgraded from, but only very rarely (once every few months a most). Now I've had three panics in the course of a few hours. The irony is that one of the reasons for upgradings was this exact problem; I assumed it would have been fixed, as it was mentioned on this list already.
I would like to file a bug report, but I can't gather all the infos asked for https://www.openbsd.org/ddb.html, as the keyboard stops working after the kernel panic (probably because it's a usb keyboard). So, here's the output I can give you:
# panic: ehci_device_clear_toggle: queue active
Stopped at Debugger+0x7: leave
TID PID UID PRFLAGS PFLAGS CPU COMMAND
*359035 78367 541 0x1002 0x80000 1 usb
216484 46276 0 0x14000 0x200 0 reaper
Debugger(d0a08f55,f54ee848,d09d62e0,f54ee848,0) at Debugger+0x7
panic(d09d62e0,dbaed460,f54ee88c,d08a0895,0) at panic+0x71
ehci_device_clear_toggle(d5d8ff00,d5d8ff00,d5a02800,0,2) at ehci_device_clear_toggle+0x29
usbd_clear_endpoint_stall(d5d8ff00,d5d8ff00,0,f54ee8dc,400) at usbd_clear_endpoint_stall+0x20
ugen_do_write(d5aa0000,1,f54eee8c,1,f54eed10) at ugen_do_write+0x2a8
ugenwrite(3f01,f54eee8c,1,d0508d09,db91169c) at ugenwrite+0x4f
spec_write(f54eedb8,db7b2aa4,f54eee74,d03cd5e9,d0bf6ae0) at spec_write+0xa7
VOP_WRITE(dba0fccc,f54eee8c,1,dbaf2c00,17a8840) at VOP_WRITE+0x42
vn_write(db91169c,db91116b4,f54eee8c,dbaf2c00,d0bf6ae0) at vn_write+0x8a
dofilewritev(db7b2aa4,8,db91169c,f54eeef4,1) at dofilewritev+0x1c6
sys_write(db7b2aa4,f54eef5c,f54eef7c,0,200286) at sys_write+0x8f
syscall() at syscall+0x250
--- syscall (number -813756072) ---
One piece of context: the uid 541 is the user _cups, under which cupsd runs.