I don't know if "stolen" is the right word, but I used to post on websites when I was younger (mostly on Neopets, shame on me), and I found a poem one day that had the same theme as mine, similar structure, and some of the same wording and lines. Of course, it wasn't my best work, and it was probably some 10 year-old kid who thought it was fun, but it taught me to never post my work online again (unless I really don't care much for it)