well, they did it pretty cleverly - blank sheet music.net. free blank sheet music basically. But the popup only appears if you press the print button and then cancel. If you do that, google ads appear in a popup. I suppose I should report them...
I know a guy who got banned because some website was showing his website in a POPUP without his knowledge - so be careful
Yet even though it's against TOS and most us use popup blockers a lot of popunders still work. These never make me want to click them so I wish people would just stop with them (although I guess they work for CPM-based advertising systems).