ah man i did it again last night, even after all the discussion here about it. i'm not sure how it happens, seems like when i'm in a hurry and have been leaving a bunch of message for others and receiving a bunch i just accidentally type up in my own message box. or maybe i'm just a klutz anyhow....
I don't use it much as the text blends in with the background way too much. :/
Anyway, the only time I leave a visitor message on my own page is if I just want a message people will notice if they try to contact me. For example: "I'm logging off for the night, all 1852601 people who won 999 billion Bells on that giveaway thread."
Every now and then I mess up and do it but for the most part I chill on someones page when I'm trading with them to avoid that haha and to give them a wifi/rating as soon as the trade is done so I can move on to the next thing I need to do.