Update failed Bad Rom

[Blake1]

Sometimes when I update phones via provisioning (using my own HTTP server and firmware.url parameter pointing to the new ROM) they fail and show this on the screen:
Update failed
Bad Rom
They reboot over and over just showing that error.

Other phones (same model, previous firmware, and network) take the update without issue, it only usually picks on one or two of them.

I seem to mostly have this issue on T40P model, but I had it once on a T40G as well. It doesn't matter what version I'm trying to update to or from, it just happens on occasion on the odd phone.

If I take out the firmware.url from the provisioning the phone does work the next time it reboots and stays on the old firmware; but this means I cannot update the phone remotely and must log in and do it manually.

Does anyone else have this issue? Any ideas of what could be causing it?