actually it happened again but i have been able to isolate the issue.
the error is below and seems to happen when i create an interface to the bridge with a vlan. that makes sense to me but, as i am new to PM and experienced with ESXi, it presents some challenges to my typical virtualised network design.
no
physical interface on bridge 'vmbr99'
/var/lib/qemu-server/pve-bridge: could
not launch network script
kvm: -netdev
type=tap,id=net0,ifname=tap100i0,script=/var/lib/qemu-server/pve-bridge: Device
'tap' could not be initialized
TASK ERROR: start failed: command
'/usr/bin/kvm -id 100 -chardev
'socket,id=monitor,path=/var/run/qemu-server/100.mon,server,nowait' -mon
'chardev=monitor,mode=readline' -vnc
unix:/var/run/qemu-server/100.vnc,x509,password -pidfile
/var/run/qemu-server/100.pid -daemonize -usbdevice tablet -name router1 -smp
'sockets=1,cores=1' -nodefaults -boot 'menu=on' -vga cirrus -k en-us -drive
'file=/var/lib/ISOs/template/iso/vyatta-livecd-virt_VC6.3-2011.07.21_amd64.iso,if=none,id=drive-ide2,media=cdrom,aio=native'
-device 'ide-cd,bus=ide.1,unit=0,drive=drive-ide2,id=ide2,bootindex=200' -drive
'file=/var/lib/vz/images/100/vm-100-disk-1.raw,if=none,id=drive-ide0,cache=writeback,aio=native'
-device 'ide-hd,bus=ide.0,unit=0,drive=drive-ide0,id=ide0,bootindex=100' -m 512
-netdev
'type=tap,id=net0,ifname=tap100i0,script=/var/lib/qemu-server/pve-bridge'
-device
'rtl8139,mac=12:F2:49:1D:15:BA,netdev=net0,bus=pci.0,addr=0x12,id=net0,bootindex=300'
-cpuunits 1000' failed: exit code 1