An increasing number of foil capacitors (metallized thin film capacitors, MTFC) are used as filters in power converters. Thus, they are constantly connected between power lines (different phases) or between power lines and ground. However, after some years of operation, severe damage cases were observed, where filter capacitors became extremely hot and started a slow process of outgasing which sometimes is even ended by catastrophic burning. Failure analysis of surviving capacitors of the same kind in neighbor phases identified air inclusions as the cause, which led to internal corona discharge. Useful corona discharge measurements, based on a modification of the well-known partial-discharge (PD) tests, made it possible to assess capacitors on their potential risk of such failures. It turned out that many, even new foil capacitors, started internal corona discharge already underneath their official AC voltage specification.