Pittsburgh is one of small number of cities that use the "burgh" spelling rather than "burg." In 1890, the United States Board on Geographic Names decided that the "H" was to be removed from all cities ending in "burgh."
Official city documents ignored this decision, and continued to the use the Pittsburgh spelling regardless.
In 1911, the Geographic Names Board finally relented. In the face of numerous complaints from Pittsburgh residents, the name of the city was officially reverted back to Pittsburgh. However, confusion over the spelling of the city remains to this day.