Yellow jackets! Actually, they're more "on" the stove than "in" the stove. The fireplace shown in my avatar is one of four in this house, this and one other being of the cooking (aka "walk-in") variety. It's part of the old summer kitchen, which I believe was added to the house with the 1775 addition, but I guess could be older (oldest part of house is 1730's).
Anyway, there's a cedar shake roof on the part of this fireplace that protrudes outside, thru the wall of the 1990's addition that now encompasses it, and yellowjackets had built a nice big nest in the little "attic" formed by that roof, finding access between two of the cedar shakes... which are probably way-overdue for replacement.
We hadn't noticed them all summer long, but with cooler weather setting in last week, they started coming into the house. They seem to have tunneled thru the bedding mortar (mud) and knocked out a spot of pointing (lime mortar), to find their way into the house thru the fireplace. Add another item to my revolving "to do" list.
I killed 10 last Friday, and 14 on Saturday, before getting out there with some foaming wasp spray to hit them Sunday morning. My work slowed them down, but didn't kill the nest, so we had a pro stop by last week. He sprayed some dust into their entrance hole, and then we were finding dead or dying yellowjackets on our floors and windowsills for about 3 days... averaging maybe 60 - 90 specimens per day. There must have been hundreds of them in that nest.
There's another nest in the chimney of our patio fireplace, but I just leave them alone. When cool evenings hit this fall, and kids still want to use the swimming pool, I'll light that so they can sit by the fire when they hop out of the pool. That should get rid of anyone still there, by then.
I've mentioned here many times, that I always have one small fire in each stove in the fall, before doing my annual sweeping. This is why, I always imagine the possibility of knocking a giant wasp nest out of a chimney, and having them come pouring out of the stove, if I do it late summer. I also imagine the possibility of finding a chimney dangerously plugged with a nest in October, if I did my sweeping in May or June, like most folks. A small fire late September, the day before sweeping, can make sure that anything in the chimney is dead or vacated, before I sweep everything clean in preparation for October heating.