The biggest mistake they made was calling it "Windows". At the time the general public had a very low opinion of MS blaming them for the slowness of their PC's etc. when most of the time it wasn't actually anything to do with MS. Basically anything named Windows was doomed from the start, they should have called it something new and fresh sounding instead, it didn't resemble Windows on the desktop at all at the time, and so another name would have made more sense, I mean "Tiles" would have been the closest actual description, but that's too bland as well... In any case, it's history now, Microsoft were too late with their OS, and Nokia just became too cumbersome and couldn't move fast enough. Microsoft might have been able to save Nokia if their phone OS had been ready a year or two earlier.
Another PR fail was the move to Windows Phone 8. Those on Windows Phone 7 were told you couldn't get 8, but instead we're giving you 7.8 What they should have done is marketed 7.8 as 8 but with some features not available on older devices. This is basically what Apple do, they give the new phone OS to everyone but not all features are available on all devices, but they call an OS with missing features on certain devices by a different name. BTW I am not an Apple ******, I have never and will never own an iPhone, the only Apple thing I've eve owned is iPod shuffle about 14 years ago.