Based on modeling and previous flight experience of Boeing 737-300s, it was believed that the cracks could not develop in this area of the plane.
Those cracks, which can develop after repeated takeoffs and landings, may well have been what caused the fuselage to fail, according to some experts.
"Now we may have to look at airplanes in places we never thought we would have to check before," said Ganyard, an ABC News consultant.
Other fuselage cracks were found and repaired on this particular aircraft during heavy maintenance in March 2010, according to The Associated Press.
Upon review of the records of the repair, ABC News discovered at least a dozen fuselage crack repairs.
Southwest now says that what was seen with Flight 812 was a new and unknown issue.
"Boeing has since identified an inspection program for this section of the aircraft," Southwest said in a statement. "Based on this incident and the additional findings, we expect further action from Boeing and the FAA for operators of the 737-300 fleet worldwide."
On Friday's harrowing flight, the first 20 minutes all appeared normal as the plan climbed to 36,000 feet. Flight attendants had just taken drink orders when the plane's 118 passengers reported hearing loud pops. Then, with the plane at approximately 34,000 feet, the roof opened up near the luggage compartment and oxygen masks dropped in front of passengers. At least one flight attendant passed out.
Passenger Wade Allemand said he almost passed out.
"Your ears instantly start to hurt really bad. You feel like you're going to black out," he said.
The crew turned the jet sharply to the right, pushed the nose down, losing more than 20,000 feet in just four and a half minutes.
Passengers had a short amount of time to get their oxygen masks on.
"It depends on what the decompression is. Sometimes it's minutes, sometimes it's two to three seconds," Lauren Jarmoszko, a flight safety instructor, said.
Loss of oxygen impacts cognition, memory and reaction time. It can also lead to brain damage.
In a rapid decompression, like the Southwest flight, the oxygen mask drops quickly. There are also instances of slow decompression which is usually caused by a small leak in a window or door seal. Passengers and crew may not even notice the first effects: fatigue, mental confusion, and dulled reflexes.
The captain on Flight 812 had 19 years experience with Southwest, while his first officer had seven years of experience, and he received assistance as typical when declaring a emergency, according to the NTSB.
Some terrified passengers clearly thought it was the end. One woman whose husband was on the plane received a text from him -- "Plane going down. Love you."
However, the plane was able to land at Yuma Marine Corps Air Station/International Airport at 4:07 p.m. Friday, said Ian Gregor of the Federal Aviation Administration.
Passengers applauded the pilots upon landing, called loved ones and waited for a new Southwest jet to pick them up and take them on their way.
ABC's Jessica Hopper contributed to this report.