Your memory is not as reliable as you might think. Things like simply changing the wording of a question regarding an event can affect how it is remembered. David McRaney at the You Are Not So Smart blog explains how it happens, and why it may make eyewitness testimony untrustworthy.
Each time you build a memory, you make it from scratch, and if much time has passed you stand a good chance of getting the details wrong. With a little influence, you might get big ones wrong.