We meet at a friend's house (C) for small group a couple times a month. Adults + kids, there is about 20 people. C lives in his grandma's old house, and she was a talented gardener - the back yard is gorgeous. He's had to do little but basic upkeep, watering, etc.
With kids playing you expect some minor trip & fall injuries. What you don't always expect is horrible blistery hive-type sores like a bad allergic reaction. C didn't think it was anything in the garden -- he has played back there all of his life.
Just in case, they tried identifying plants on the internet and didn't find any of the usual suspects (poison ivy, etc). Then another kid got the blisters. They took samples to a local nursery and found the culprit: Myrtle Spurge (Euphorbia myrsinites). It's considered a noxious weed here, and not legal to plant/sell - but these were put in many years ago.
This weekend will be plant removal - they're all over the xeriscaped yard - and they're just thankful it wasn't ingested by their son and dog who have been playing back there for the last year.
....In case the link doesn't work, here's what it looks like....