Imagine a scenario where you know beyond a reasonable doubt that somebody did something wrong, but you couldn't prove exactly what it was. It could have been A or B. But you cannot prove it was A and you cannot prove it was B.
EDITED TO ADD A SCENARIO:
Imagine a marijuana salesman is being charged with fraud. His accuser states that he has been sold fake marijuana which constitutes fraud. The selling of marijuana is also considered a crime. Multiple witnesses attest to the fact that the marijuana seller was purporting to sell marijuana. The evidence has been smoked. There is no way to determine whether there was a fraud or whether there was an illegal sale of marijuana. But the number of witness reports puts it beyond a reasonable doubt that one of the two occurred.