Sponsorship: Los Alamos National Laboratory. FPGA designers are becoming increasingly aware of fault tolerance issues in modern FPGA designs, especially designs destined for a radiation environment. We classify errors due to upsets within the configuration bitstream into two categories; namely, persistent and non-persistent. Persistent errors generally cannot be tolerated. However, non-persistent errors can be tolerated in certain types of designs as long as they are properly accounted for. We discuss situations in which non-persistent errors are acceptable, and describe a technique for the detection of upsets causing persistent errors within the configuration memor of an SRAM-based FPGA.