Scenes are rarely destroyed. They are diluted, a little at a time, by people who each did something reasonable.
The ways it happens, as the manuals describe them:
- Too many people. Every person present adds their own fibres, prints and DNA, and removes some of what was there. Presence is a cost; it needs a reason.
- Movement before recording. An object moved before it is photographed in place has lost the one thing that could not be recovered — where it was.
- Transfer between scenes. The same gloves, the same boots, the same vehicle carrying material from one location to another. Guidance on this is blunt: change protective equipment between scenes, and treat a suspect and a victim as scenes that must never be attended by the same person in the same clothing.
- Packaging that damages. Wet material sealed in plastic grows mould within hours; biological traces are destroyed by it. Paper for anything damp or organic, and dried before sealing where the guidance allows it.
- The unrecorded good deed. Somebody opens a window, turns off a tap, covers a body. All of it may be right; none of it is harmless unless it is written down.
Source. UNODC, Crime scene and physical evidence awareness for non-forensic personnel, chapters on contamination and packaging; NIJ NCJ 178280, sections on scene documentation.