There is a fine distinction between a case where there is not enough evidence to convict someone and another case where a complainant blatantly lies. The latter is a false accusation the former is not a false accusation. it defies logic to assume that in all instances where rape (or any other crime for that matter) took place, that it would result in a successful conviction.
Why is this tagged with "false accusation"? The issue is not false complaints but lack of disclosure of evidence. An unproven allegation is not the same thing as a demonstrably false accusation.
