Legal Question in Criminal Law in California

Re: Legal witness

Generally, evidence of "prior bad acts" is inadmissible to prove what the defendant did in a current case. But such evidence is admissible in order to prove other matters, such as identity or plan. Thus, if the defendant says the police arrested the wrong guy but a prior victim can testify that he did exactly the same thing to her, the evidence will usually get in.

Additionally, California law makes evidence of prior sex crimes admissible in circumstances where it otherwise would have been excluded for precisely the reasons you mention.

There are many different ways such evidence might be offered and many bases upon which a judge could decide to admit it or exclude it. One factor the judge will take into account is the possibility that the prejudicial effect this evidence will have on the jury will substantially outweigh its probative value.