Surprise, surprise! One family went into full panic mode when playing with a raggedy old tennis ball it found near the Swan River in Western Australia.

Make no mistake: This wasn't a case of second-serve jitters or a string of untimely unforced errors.

Rather, Kristy Edgelow said her "older son noticed something moving inside" the ball, according to PerthNow.

The whole situation was perhaps a little disconcerting, maybe even creepy, until they noticed what exactly was inside the ball.

"We got the fright of our lives when this little fellow popped his leg out and we realized it was a blue-ringed octopus," Edgelow said.

These creatures look innocuous enough, but their bite is, well, a helluva lot more destructive than you'd imagine, given that the animals don't grow larger than a golf ball.

In severe cases, patients will experience vomiting, respiratory paralysis and even death.

No antivenin is available.

"My heart was racing that's for sure...so please watch out," Edgelow said in a Facebook post.

The blue-ringed octopus is generally a bottom-dwelling species, but according to aquariumofpacific.org, it seeks shelter in empty seashells and discarded bottles and cans.

And, apparently, damaged tennis balls.