How is it possible to be found guilty in a sex crime when there is absolutely no evidence

People who say there is no evidence always scare me a little.  How do you know there is no evidence??  There is always some evidence.  First, at some point a child reported something.  Evidence.  Then the officer works to confirm the childs statement by corroborating facts.  Evidence.  Then, there may be things other people saw or noted.  Evidence.  There is always evidence.  Wheyher that evidence is sufficient to convict a person of a serious crime shuld be the question.  Thats where a skilled defense attorney can be invaluable.  Challenging their "evidence" and attacking their case.