Single, high-energy ions can induce large charge loss from Floating Gates used as basic storage elements in nonvolatile memory cells. The charge loss greatly exceeds that calculated by using conventional models based on generation and recombination of charge. Further, the ion locally damages the tunnel oxide, leading to degradation of retention properties of the Floating Gate over long times. This mechanism has important implications to design fault tolerant Floating Gate memories which should be operated in radiation harsh environment.