Try this in any of your web app and you'll find something interesting. Make sure you set a break point in your page_load event. Well, the finding is very obvious once you see it, but its something that you might never guess off the top of your head. Basically, background="#" property causes the page to reload, it tries to find the image you specified, in our case "#", which is converted to current URL and causes page to be called twice. This might not have much effect on functionalities, but there might be cases where it does have some impact.
First obvious impact is, it causes the page to post for no reason. And secondly if you are performing any operations during Page_Load it might be performed again. Well, I learned this the hard way, trying to understand why my page refreshed twice. Suspected, I had a flaw in the complex page architecture I put together. But was dumb founded when I traced it to this.
So lesson of the day, make sure you dont' set your backgrounds to "#". And there shouldn't be any reason why you'd be doing this, well unless it's a typo.