Single-event upset was investigated in high-density flash memories from two dfferent manufacturers. Many types of functional abnormalities can be introduced in these devices by heavy-ions because of their complex internal architecture. Changes in the stored memory contents sometimes occurred, even when devices were irradiated in a read mode with the internal charge pump inactive. For one device technology, unusually high currents were observed during post-irradiation cycling that were high enough to cause catastrophic failure.