Rather more common than one would think :-)) There are a number of particularly disgusting looking fungal organisms that can pop up periodically.

I poked it with a stick expecting it to be crusty and it was this god awful yellow goo inside. I ran screaming to the house and hid under the bed.

LOL!!!

I see "vomit" frequently in or oozing out of any of my compost bins.

I've never had a garden where slime mold didn't show up, and I've had 12 gardens.