When dumping the data from a rom, an overdump occurs if more data is read than the capacity of the rom. This tends to happen with console cartridges, where the amount of actual data is variable, but the number of address lines is fixed. Generally, the extra address lines are ignored by the cartridge, so the read appears valid, but will be a copy of data at (read address MOD actual size of cartridge).

For emulation, rom managers like GoodNES or romcenter will report overdumps, but it is up to the user to delete them.