I think I found a bug in version 12 and hopefully it's already known and fixed in an update soon to be released, but just in case, here's the scoop.
If a field has validation which uses Validate by Calculation and then the field is edited on a layout where it's in a selected portal / preview pane scenario, (ie, the user clicked a portal row to select a related record and the related record's field are displayed outside the portal for viewing or editing.) When validating the field, if the field fails validation, the user gets an error saying that the field has validation but that the field isn't on the layout and the user should edit the record elsewhere. Then it gives an OK button which allows the invalid data to remain in the now committed related record. screen shot of error message attached.
Hope this can be worked out. in the mean time, I'll plan on not allowing editing of validated fields in a preview pane.