In Rolf Harris’s case the prosecution relied on what is known as ‘similar fact evidence’.
The fundamental safeguard in our system of justice has always been the presumption that defendants are innocent until they are proved guilty. The ‘similar fact principle’ establishes the conditions under which factual evidence of past misconduct of accused can be admitted at trial for the purpose of inferring that the accused committed the misconduct at issue.
It used to be the case that for a prosecution to use fact evidence against a defendant that it would have to be ‘strikingly similar’ to the case in question: For evidence to be strikingly similar “…The similarity would have to be so unique or striking that common sense makes it inexplicable (strange) on the basis of coincidence (chance/ luck/ accident).”
Over recent times the safeguards over the use of similar fact evidence has now been eroded. There is an excellent memorandum on the point by the late Richard Webster.
The similar fact principle has been eroded to the point where one allegation can be supported on the basis of multiple uncorroborated allegations, all of which might be false.
Where witness statements are demonstrably false, or else where there is a total lack of any corroborating evidence (such as whether an event actually took place, or whether the alleged complainant was actually present), or even where the allegation is not proven beyond reasonable doubt, how can it then be used to corroborate another doubtful statement?
In the case of Rolf Harris, none of the accusations in themselves amounted to anything. Ie none of them could be proven, all of them were doubtful, and one of them definitely didn’t happen. Adding them all together therefore can’t make them amount to anything either (ie lots of nonsense is still nonsense…).