A flash game I built in high school (about 7 years ago) went from an average of $30 per month on AdSense to $500 the next month, to $3,600 the month after that due to an unexpected surge in traffic for a keyword for which it ranked #1 on Google. The $3,600 month happened in May, and it has since dropped steadily to about $1,500 a month now. Regardless, it was a nice surprise since the game was just sitting there from when I spent a weekend building it a long time ago.