A character is in a very nature oriented setting, with all kinds of wild plants surrounding them. They're doing whatever it is they planned to do, and at some point pick up or walk into an ordinary looking plant. After a small amount of contact with it, someone who is more knowledgeable about nature will casually inform them "That's poison ivy" (or poison oak, the plants have the same toxin).

Examples:

• The early camping episode on Hey Arnold ended with Big Bob Pataki rushing through a small bush to get back to his campsite after a disastrous hike. This bush, of course, was poison ivy, causing him more misery.
• Early in the movie Coraline, the title character brushes some leaves off a stick and uses it as a water rod. She meets Wybie, and has a fairly long conversation with him, and at the end he throws in that the stick she's holding is poison oak.
• After a huge fight and emotional breakdown with an unrequited love interest in a forest, the title character of Malcolm in the Middle dries his tears with some leaves he found. The girl he is with tells him he's using poison oak. We see a rather disturbing shot of his face a little later, which resembles the character No-Face in Twisted Metal Black.