form_set_error and ahah callbacks

Hi,

I have the following strange situation: I have a form with some ahah logic. Furthermore there is a validation function for this form which calls form_set_error in case there is a problem with the data of the form.

1) If the user enters same invalid data but does not invoke any ahah actions then the error is reported by the validation function. Then the user can correct the data and everything is ok.
2) If the user enters same invalied data but has triggered same ahah actions during the data entry, then -again- the error is reported by the validation function. But then, when the user corrects the data and hits the submit button, the ahah callback function is called (and not the normal submit function of the form) and then some json code (from the ahah callback function) is printed in the browser window.

Does anybody know why this is the case? Why doesn't the situation in 2) work as expected?