Mount Dissapointment is located in California’s San Gabriel Mountains.

From Wikipedia:

It was named “Disappointment” in 1894 when USGS surveyors in the Wheeler Survey sighted it from the Santa Susana Mountains, believing it to be the highest point in the immediate area, decided to use it as their next triangulation point. When they reached the summit, however, they discovered that San Gabriel Peak half a mile (0.8 km) to the east was 167 feet (51 m) higher and it was a disappointment