If you're using Simple Popup
http://extensions.joomla.org/extensions/style-a-design/popups-a-iframes/17214
I thought I had the settings in it's plug-in control panel set correctly so that the user would only see the popup once each day. However, it kept showing up every time I loaded the page with the popup.
The solution was to call the popup with the following code:
{simplepopup name=MyPopUp cookie=1}