Forward Error Correction (FEC) can effectively eliminate data loss (frame loss) in systems with moderately high Bit Error Rates (BER) of 1E-4 or more. However, in real systems, errors sometimes occur in bursts and can lead to FEC failure and non-zero Frame Loss Ratio (FLR). This presentation will demonstrate a real-time oscilloscope combined with FEC-aware tester. This solution will be shown to trigger on, capture and debug burst errors, or uncorrectable FEC events that can lead to unexpected frame loss in 400GBASE-DR4 and 400GBASE-SR8 devices. New solutions for 800G OSFP devices will also be covered.