This feature is currently in walk.
DISCO validates overlay files before initiating the overlay process. If the overlay file fails validation, then the overlay card displays a downloadable validation report containing details of the failed validations.
To download an overlay validation report, on the overlay details card, click the more options button and then click Download validation report.
The validation report contains a Summary tab and a Data Load File tab. The Summary tab contains a description and counts for each failed overlay file validation. For each failed validation, the Data Load File tab contains samples of the rows in the overlay file that failed the validation.
Validations help decrease the probability that an overlay will have exceptions, but they do not eliminate the possibility altogether.