It's spelling bee season! The National Spelling Bee is still going on at the time that I write this. But I'm confident that none of the finalists would have trouble with any of the words on this map.
Google Trends measured searches for "how to spell X" -- the X being a mispelled word. It made this map illustrating its findings. You can see a larger version here.
For Massachusetts, the hardest word is Massachusetts. For Arizona and New Hampshire, it's diarrhea. North Carolina struggles with pneumonia. California, Idaho, Indiana, and Connecticut have trouble with desert.
-via Debby Witt
And makes you wonder what's going on in Arkansas and Utah.
In my job I handle global content and the UK vs. US spellings often come up as an issue to those not in the know (aside from the obvious color/colour and theater/theatre, etc). I've been in the midst of many discussions about "misspelled content" due to people not being aware that they are looking at Euro English vs. US English. "Cancelled" is always a big one that many people are unaware of, which is why I think it hits so many times on this list. People have a recollection of seeing it both ways (correctly in context) at times.
But "gray"? I think that may be just people arguing about "gray" vs. "grey." The 50 Shades book did not help there.