After Elaine throws her back out, she calls Jerry and says that she is trapped under the mattress. However, when she falls, you can see clearly that the mattress doesn't hit the ground, as it is blocked by the wall.
When George (Jason Alexander) wakes up from underneath his desk, he places a copy of a People magazine with Mel Gibson on the cover in a compartment under the desk. However, the magazine can later be seen on top of the desk when Steinbrenner, voiced by Larry David enters George's office.
The bomb robot didn't have the chainsaw when it entered the office.
When Mr. Wilhelm enters George's office and asks for the American League directory, the shot of George closing the book is played backwards (This error is present in the DVD version).
The bomb squad would never carelessly chop into a desk that may contain a bomb. The bomb would first have to be located before slicing up the desk. A physical crew would go in first with x-ray machines to find the location and find out if there are any triggers on the bomb that would be denoted if they opened the desk in any way.
Public pools are cleared out for classes.
To move the fridge and stove Jerry would have needed an electrician to move the outlets.
When Kramer is swimming in the East River the lack of movement mid-body shows that he is being supported from underneath.
Jerry has new kitchen cabinets installed. One section is supposed to be secured to the ceiling. George hits this section, and it noticeably swings back and forth. This is due to that they are most like hung from set rigging. Also, you can see the bottom side of this section of cabinets, and see that it is just a hollow shell designed to look like kitchen cabinets.
A scene in Jerry's apartment reveals the bottom of set lights. It is in the scene with Jerry and George, right after Elaine and Kramer leave.
Hal tells Elaine that Twinkies aren't cooked. They, in fact, are.