This is how the official story has Flight 77 flying into the Pentagon:
Notice it's fuselage strikes between the 1st and 2nd floors and it's vertical tail (or stabilizer/rudder) extends above the 3rd floor and it's right horizontal stabilizer should have contacted the wall between the 1st and 2nd story windows:
Now here are some different accounts from witnesses as to what happened to the tail section (note that there are no witness reports of any part of the tail section flying over the building when the plane hit):
"AP reporter Dave Winslow also saw the crash. He said, "I saw the tail of a large airliner ... It ploughed right into the Pentagon." - Guardian (09/12/01)
Statement from Penny Elgas
"The plane seemed to be floating as if it were a paper glider and I watched in horror as it gently rocked and slowly glided straight into the Pentagon. At the point where the fuselage hit the wall, it seemed to simply melt into the building... At that point, the wings disappeared into the Pentagon. And then I saw an explosion and watched the tail of the plane slip into the building." - Smithsonian Institute
"The World Trade Center attacks were followed by a similar attack on the Pentagon. An airliner crashed into the building, causing a major explosion and fire. Witnesses saw a tail section sticking out of the building." - 720 WGSE
"The plane exploded after it hit, the tail came off and it began burning immediately. Within five minutes, police and emergency vehicles began arriving," said Vin Narayanan, a reporter at USA TODAY.com, who was driving near the Pentagon when the plane hit." -USA Today (09/11/01)
A Boeing 757's tail section is huge and should have either left a gash in the wall if it partially entered, or left a very noticeable mark on it if it sheered off and crumpled against it before bouncing off, or from being obliterated into a million pieces from hitting the recently retrofitted wall at a blistering speed of 530 mph. However, you can clearly see that none of that happened:
Also notice there is a piece of column still hanging from the 3rd floor. Columns are usually made of long thin steel rebar incased with concrete, so this piece should have been bent back into the building in the direction the supposed plane crashed in and not dangling as if it were attached by strings:
You can see from this early photo taken before the roof collapsed (which was about 30 mins after the "crash" at about 9:38 am) that there is no large plane debris from a tail section or any other section from a 757:
I have yet to come across any photo or video that shows what looks like debris from a 757's tail section or any photo showing a mark on the Pentagon's facade where the vertical and/or right horizontal tail section smashed up against.
Tail sections usually survive plane crashes since they are usually the last to hit the object the plane crashes into and because it's at the rear of the plane:
If you get into a debate with someone who still believes in the official story that a Boeing 757 hit the Pentagon, just show them that there is no evidence of a 757's tail section hitting the wall and sheering off, or obliterating against it. Also, mention that tail sections from planes do not simply disappear without a trace.
The missing tail section from Flight 77 is the best smoking gun that a Boeing 757 did not crash into the Pentagon.