Danielle O'Brien was an air traffic controller at Dulles on 9/11. She spotted a fast-moving blip on the radar as it approached the Pentagon, and her recollections were later used to suggest that this may not have been Flight 77 at all.
In The New Pearl Harbor (updated second edition), for instance, David Ray Griffin uses this quote (amongst others) as part of the justification for questioning the identity of the plane:
The same quote appears in many other places, often in an attempt to cast doubt on what exactly hit the Pentagon. The site asile.org, for instance, came top of the Google list for one search, in a page entitled "EYEWITNESS ACCOUNTS: Boeing 757 or military craft?"
Whatever you think of the theory, the asile.org does at least include a quote from O'Brien that Griffin and many others ignore:
O'Brien wasn't saying a 757 couldn't do what she'd seen, rather that it was being flown in a dangerous way. As the hijackers were about to fly it into a building this seems unlikely to have been a concern.
But is that just us offering a counter-interpretation of her words? No, because O'Brien has spoken out about this before. Here's a letter she wrote, responding to the interpretation of her words by Thierry Meyysan, who first popularised the "Pentagon was hit by a missile" theory:
No matter how much Dr Griffin and others may want to pretend otherwise, O'Brien seems to have no problem at all in accepting that what she witnessed was not a military plane, just a commercial airliner being flown in a reckless and dangerous manner.