I have 62 icons on my Windows 7 Desktop. (Only one monitor.) (Windows Classic theme.) I always enable "Auto arrange icons" and "Align icons to grid" (and never disable "Show desktop icons"). With Windows XP and prior, whenever I downloaded a file to the Desktop (using Firefox, with its default save-to location set to Desktop), Windows added the icon for the new file at the END of the Desktop icons: below the lowest icon in the right-most column of icons if possible, or else at the top of the next column. Now, Windows 7 adds the icon for the new file in the FIRST column of icons, second position (below Computer), and all the other icons get pushed down by 1 (with wrapping to the top of the next column). I didn't ask Windows to insert the icon near the top; I expect it to add it at the end, like before. What happened? How to fix it? (At times I think I triggered it by using "Show hidden files" and turning off "Hide protected operating system files".)
This is an OLD problem. (Maybe Vista did it too. I didn't notice.) I am disappointed that I never stumbled onto an answer. I am disappointed that I cannot FIND an answer. I found other posts of the question, but it was not phrased well, enabling answerers to misunderstand the question and post irrelevant answers.
Right now I cannot clarify which provocations cause the problem and which do not, because my Windows is, for the moment, doing the right thing. (Murphy's Law.) Some other provocations that I would test are: If I do "Save Page As…" or "Save Image As". If I download or save from a different browser. If I open an Explorer window, navigate to "Desktop", "%userprofile%\Desktop", or "C:\Users\Public\Desktop", and create a new file or folder. If I open a command prompt and type "md Desktop\x". If I save from a different app, like Notepad2 or WordPad.