Eyewitnesses will often add in things like grenades going off, helicopters, bombs, extra gunshots etc. Especially if they talk to each other and share their stories - people will often incorporate suggestions from other people of what happened even if they didn't actually witness it. There have been some interesting experiments done on this. It's great fodder for conspiracy theorists as you can always get some odd eyewitness accounts of major events. Even minor events can be difficult for the police to sort out from witnesses. Not to mention all the people who just make stuff up to get attention, and the people who swallow it unqestioningly to be part of the drama.