Every time you fail to convict a guilty person, you do the same. That happens FAR more often.
Humans are not perfect and it is insanity to demand perfection from human institutions such as the justice system. What can be demanded is for every reasonable doubt be addressed before conviction.
Might want to read this recent USSC case about a state and judge which prevented a defendant from presenting a viable defense in his criminal trial.
http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=US&navby=case&vol=000&invol=04-1327