The ship capsizes right at midnight. The combination of the length of the emergency lighting staying on, the rate of sinking and the story being acted out in "real time" makes it impossible for it to be daytime when they are rescued.
Linda Rogo had to change into her husband's shirt because she didn't wear a bra and couldn't climb in her gown. However, when she is in the smokestack about to climb the ladder, a bra strap is visible. In the dvd commentary Stella Stevens acknowledged that she saw this error in the rushes and pleaded to have it removed but to no avail.
As Terry falls to his death in the encased light fixture, party guests are visible standing perfectly fine on the "ceiling"; in the next shot, all of the guests are sprawled about the "ceiling" in different stages of injury and death.
When Belle decides to go underwater and save everyone's life, the camera cuts to a closeup of her. She is fingering the swimming medal around her neck. But the surrounding shots have her hands clasped in front of her.
The pressure inside the ship would have been much greater than the outside pressure. With all the doors open inside the ship, when the torch first cuts through the hull the air would have rushed out and the ship would begin sinking faster.
As the survivors led by Gene Hackman make their way toward the engine room they get dirtier and dirtier, except for the exposed legs of Stella Stevens and Pamela Sue Martin, which remain silky smooth and dirt-free.
Just after the capsizing scene in the dining room, many passengers are seen clinging to the dining tables, which were bolted to the floor (now overhead). Many chairs had fallen, but several are still attached to the floor by ropes which were used to hold the chairs in place during rough weather.
Reverend Scott makes no effort to resuscitate Belle after she dies from her heart attack. While it's possible she was beyond saving, the fact remains that Scott never tried CPR or anything else that may have saved her. (NOTE: As CPR training of the general public had just started on any large scale during the same year as this film was released, it is probable that the character of Reverend Scott would not have had even a rudimentary understanding of the process of CPR, particularly because the world's first mass citizen training in CPR took place in Seattle, Washington in 1972, and the setting for this movie was in the North Atlantic as the ship was traveling from New York to Athens, Greece, so most of the characters would most likely not have come from the Seattle area in the first place.)
In the opening storm sequence, the point of view forward over the bow from the bridge shows the horizon canting off-level as the ship rolls, which would not have happened in reality. The horizon would remain level. The error occurred during filming with the miniature ship, when the camera was canted left and right above the model to accentuate the rolls of the ship in the storm, when it should have been attached to the model itself. This would have shown the correct perspective of the ship rolling in the storm, with no movement of the horizon ahead. The film's director acknowledged the goof in a chapter about the film in his autobiography.
As the ship is tipping over, a man in the dining room falls onto the Christmas tree and grabs onto it. Although the floor had become one of the walls, gravity is visibly still pulling him towards it. This is because although the dining room set could tilt on hydraulics for more serious angles, the camera was simply tilted.
The goof item below may give away important plot points.
When Reverend Scott falls into the fiery water, he is immediately presumed dead. But since the burning fuel would be floating on top of the water, he would not even have been burned, passing so quickly into the water. He should have been able to swim to the edge where there was no fire, and climb out.
The group makes their way to the rear of the ship, because the hull of the ship is said to be "only 1-inch thick" near the propeller shaft. Yet when the rescuers cut a hole in it and the survivors climb out, it is clearly only a fraction of an inch. Barely an 1/8", even.