The Monopoly square Marvin Gardens is unusual because, unlike other squares on the American board, there is no real Marvin Gardens in Atlantic City.
It is in fact a misspelling of Marven Gardens, which does exist in Atlantic City. The name comes from the area's location on the borders of Margate City and Ventnor City.
The mistake was present on Parker Brothers' first edition of the game and has remained there ever since, despite requests from Margate City commissioners to change it.
People are so familiar with the incorrect spelling from the Monopoly game that it is constantly used, even in newspapers and reference works, to refer to the real place.
In an effort to publicise the correct spelling, the Margate City board of commissioners set up a publicity stunt, officially changing the name to the incorrect spelling for just one day (Jan 21st, 1975). Alas, they had no success with this, and the misspelling will likely haunt them forever.
It is perhaps worth noting that it's not only the American version of this square that has suffered typographical errors.
On the British version of the Monopoly board, this square is called Piccadilly. However, the property card shows the rent for no houses as £22, the same as its fellow yellow cards. Apparantly this anomaly was not unique to the British set; however, while it has been altered on the American edition, it remains in the British set, making the yellow group the only coloured set to have the same rent on all undeveloped properties.