As the ground is splitting, it travels in a northerly direction. The water fills behind it. From the character's (and camera's) point of view toward the end, this is right to left, which means they should be on the island (right is south). The last time they show the characters, they "zoom out" to show them on the mainland.
The Space Needle falls towards the side with the steps leading up to the base, which is the north side. It falls along a street in between brick buildings. The Space Needle actually has a football stadium, roller coaster, and an opera house to the direct north of it.
When Dr. Samantha Hill and FEMA Director Roy Nolan are placing the warheads at the six sites for detonation, the computer screen identifying the geographical location of the Washington State warhead actually has the Washington State warhead being placed and detonated in Oregon.
The San Andreas fault is incapable of suffering an earthquake measuring 10.5 on the Richter scale (100 times as strong as the largest earthquake ever recorded). The Richter scale measures strength as a function of fault length, and the fault in question is far too short to generate such a massive jolt (which would be felt all over the world).
A 10.5 earthquake as represented in the movie, would actually be much larger than depicted. People would not be able to walk around so freely as they are doing (at a 10.5, the levels of sight and sound would be distorted). Damage would also be total, damaging much more than shown (the destruction would also reach areas as far away as Michigan or possibly even New York).
When Dr. Jordan Fisher is in the helicopter reporting water temperature changes at Lake Mead, he is giving figures over a hundred (indicating degrees Fahrenheit). Scientists, however, generally use the Celsius scale in which water boils at 100 degrees and cannot get hotter and remain liquid except under pressure. Furthermore, the lake surface is shown as already boiling, which, on the Fahrenheit scale, occurs at 212 degrees, not at the 95 - 107 degrees reported in the dialogue. It's wrong in either scale.
The San Andreas Fault would not split apart, since it is a transform plate boundary, when two plates grind past each other. Only a divergent boundary would split apart as shown, and those only occur in the ocean apart from an area on land where two plates are pulling apart, this being the Great Rift Valley in Africa.
During the movie characters continue to use cell phones in disaster areas. In a real disaster they would not work however. This is because cell phone towers, which relay the signal, would be out either because they were destroyed in the disaster or because the power would be out after the disaster.
On the opening shot of City Hall in San Francisco, it is a spiraling aerial shot with all of the automobiles going in a forward motion. When the scene in "City Hall" is over and the spiraling aerial view is run again, it is the SAME aerial view of city hall run in reverse (as can be proven with the fact that all the cars passing long the street are running backwards in the reverse motion).
The goof item below may give away important plot points.
During the earthquake you see fracture lines start at the base of the Space Needles legs and go up. This is followed by huge chunks of concrete falling as if the Needle were made of concrete. But the Needle is in fact made of steel.