A witness’s memory can be manipulated or distorted after the fact either intentionally or unintentionally. This is why it is very important to ask questions in a way that is both careful and ethical.
An investigator could easily manipulate a witness’s memory after the fact so as to try to get the witness to testify to what the investigator wants. This can be done, for example, by leading the witness with questions. An investigator might, perhaps, show the witness a picture of the suspect and ask “is this the man” instead of giving the witness a group of pictures and asking them to identify the man they saw at the scene of the crime. If the witness does not identify the suspect from the picture, the investigator might ask them if they are sure and might repeatedly bring up the subject of the suspect. This can encourage the witness to “remember” that the suspect was there simply because they want to make their memory conform to what they are being told are the facts.
Memories can also be unintentionally distorted. An investigator might do this by carelessly phrasing questions badly. The investigator might say “which hand was he holding the gun in” instead of asking what the suspect had in his hands. Even if it is done unintentionally, it influences the witness to “remember” things in a certain way.