Can you call it an Error? Montana should be flat. This one is from an older auction, has more detail in the stamp.
I'm surprised we don't find more in circulation, but I guess they have all been put in a jar or drawer somewhere as a novelty.
Circulating 2020-W Quarters Carry ‘V75’ Privy Mark for End of WWII By Mike Unser - April 6, 2020 Ten million quarters from the United States Mint facility at West Point will enter circulation this year. Along with the West Point Mint’s "W" mint mark, they will carry a special "V75" privy mark to honor the 75th anniversary of the allied victory in World War II. I haven't even seen a "W" yet.