This make sense when you mistakenly add a pci device on a -s -S scenario, like the scenario described on the following bug:
https://bugs.launchpad.net/qemu/+bug/544367.
When ACPI-based hotplug support is present on the guest and we run pci_del with the force option, the hotplug events will still be generated to the guest and the guest still will trigger the EJx event, which will end by calling pciej_write() on qemu side. This function will do nothing on a -f and pci hotplug support scenario, as the pci device was previously removed by pci_del.
Thanks!
Marcos