For the happy man! - Collected writings DEPRESSION: Ed Atkins

Or... I suppose. False positives and contamination by
subsequent handling or nearby objects (e.g. the mixing of blood
from victim and attacker), for example, are problems owing to
the presence of many common substances and the necessity
of human involvement in the collection of trace evidence (if
only robots were sufficiently developed). Both can occur with
DNA traces and fingerprints. Partial fingerprints are even more
vulnerable to false positives. Samples from accidents or crimes
should therefore be protected as much as possible by enclosure
in a sealable container as soon as possible, after an incident is
under investigation.
Or the locked-room... In which a crime – almost always
murder – is committed under apparently impossible circumstances.
The crime in question typically involves a crime
scene that no intruder could have entered or left, e.g., a locked
room. Following other conventions of classic detective fiction,
the reader is normally presented with the puzzle and all of the
clues, and is encouraged to solve the mystery before the solution
is revealed in a dramatic climax.
[...]