Normally user error, where a index is supplied to a disk read or vice versa is detected via the record format crc check, but that is a bit obscure, it would be better if the error explicitly said the file supplies is not an index when it should be etc. Also the record format crc can be missing, in which case an attempt will be made to actually read the incompatible file with unpredicatable results (usually results in a more obscure error).