An unintended show
I was working at a Japanese university in the late 1990s, teaching a language class. Using the internet for language teaching was still relatively new, so I was constantly on the lookout for good suggestions.
My class was in a computer lab that was set up so that the students sat in pairs with their own computers and in the middle between them was a monitor that could be used to mirror my computer.
I'd set them a research task and was using my computer to explore ideas for web design. I came across a site that had a list of award-winning web-design sites, so I clicked on one -- and was rewarded by a porn site full of images of naked bodies in various poses, all faithfully reproduced on the students' monitors. I quickly clicked the "close" button on the web browser and discovered -- pop-unders, cascading down the page faster than I could close them. I finally hit the button that shared my screen and ended the unintended show.
The students, mostly 20-year-old women, were highly amused at my obvious embarrassment, and I learned a valuable lesson.