One of the things that seem to hold in most ancient legal systems is
that they don't handle disputed facts very well. That is probably why
secret murder is a capital offense in Norse law, while public killing
is usually mediated. In the back of one of my Penguin editions of the
Icelandic sagas it claims that most of the work in a legal battle was
establishing which court had the juristiction, but that the facts were
seldom in dispute. When they were, I expect that they appealed to the
ultimate judge (ie, trial by combat).
More often, they probably voted to reconsider at the next Thing, to
see if more evidence had turned up. Being caught destroying evidence
was probably sufficient grounds to prove the other side's claims.