In Safari, running in full screen mode, if a link opens a file (eg an Excel workbook), sometimes the Excel (non full screen) window will open over the top of the Safari full screen window. When that happens it's impossible to get access to the Excel window, as clicking on it will jump to the desktop "space" (where the window should have opened?). The only way out of the problem is to move to the Safari full screen "space" and take Safari out of full screen mode using the button in the top right corner (Esc doesn't work in this case). After that I can access both Safari and the Excel window no problem.
This doesn't always happen, but it has happened enough times to be annoying, and also happens with other apps (eg Preview), not just Excel.