One time I pulled in the driveway where I was house-sitting, and the car puked atf. I'm like WTF. So jack the car up - bumper jack, get stump under it so its safe, nothing broken or leaking.
slant6 '75 Valiant. Never found anything wrong and it never did it again. There's was nothing about the drive that should have caused it to puke through the vent - but that's must be what happened. weird.