This is what happened the first time I used Wyeast 1084 with a 'small' starter (about 1L of liquid, including yeast) in a honey porter... This is after 6-7 hours of activity...
Ever since then, I've made sure that either I'll be awake for the first 8-10 hours that from when the yeast is pitched or I install a blow-off tube for the first two days of fermentation. I really don't want to come home to find something like that again...
BTW, this is what the airlock looked like after 6 hours (from when the yeast was pitched)...
It was completely post-lag after 2 hours... Love it when starters do their jobs.