Disneyland opened in 1955 with much fanfare -- and a great many problems, causing opening day to be referred to as "black Sunday" by cast members. Some of these problems included:
Because of a plumber's strike during construction, Walt had to choose between having mechanical elephants in the Jungle Cruise ride squirt water out of their noses and having enough bathrooms in the park. Walt chose the elephants.
Some of the cement in the park had been set over night and was still wet on opening day. A guest who foolishly brought her chihuahua dog to the park (apparently because she didn't understand exactly what kind of "park" this was) became hysterical when she discovered that her pet was missing and its leash was embedded in cement that was quickly drying in the morning sun.
The park was overcrowded because a talented forger had created thousands of fake tickets and was selling them from his small home overlooking Santa Monica bay. For years after this, people referred to buying inexpensive items of dubious quality as "shopping at the bay." Almost half a century later, the term was immortalized when "The Bay" became "eBay."
Because there was nobody counting how many people boarded the River Belle, the riverboat was filled way beyond capacity. It sunk at the far end of Tom Sawyer Island. There were (thankfully) no significant injuries, but the boat was so large and sank so deep that it was impossible to raise. Years later, filmmaker James Cameron went down in a submersible and filmed the wreckage as part of a fascinating Imax film. The River Belle was replaced almost exactly a year later by a sister ship, the Mark Twain, and a few pieces of wreckage that floated to the surface were used in the building of the River Belle Terrace restaurant.
At one point in the day, Fantasyland had to be closed due to a gas leak. This incident lead to a rather rude joke being told at Grumpy's expense for years to follow.
On the morning of opening day, Walt Disney took a quick spin around the park and was disappointed that there were so many areas left incomplete or unfinished. As a last-minute fix, he had name tags printed for dozens of employees with Latin names on them. The idea was that the fancy-sounding names would give the impression of a park filled with foreign intellectuals and distract guests from other problems.