If you are working on GUI for web dev or whatever, there are a few annoying things you should be aware of:
1. If you have drop down menus that expand on mouse hover, please make sure there are no other buttons that can be hidden below the drop down menu. There is a risk the user might hover the menu while trying to push the button, pressing whatever is in the drop down menu instead of the button.
2. Please make sure your site/interface is fully loaded before presenting it to your user. Use a loading div on top of your page and then use a onload() event to hide/remove it once the site is fully loaded. This avoid stuff moving around while the user is trying to interact with the interface.