This must be random or situation-specific. I've run Win7 for over three years now and have never seen this issue. FWIW, my setup was factory-installed and was not an upgrade from an earlier version of Windows. I have seen bizarre issues on upgrade systems, which is why I never go that route.
I have seen utilities to 'Repair Desktop Icons' that work by deleting the icon cache and forcing Explorer to rebuild the icons. I believe it was a feature in the TweakUI package. I have used it in WinXP to correct cases where an app had an icon for some other app, but that was a long time ago and I have needed to do that since.
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