If the device detects a bad command it will send back 2 bytes to the PC:
0xFA,
followed by the bad command byte.
If the data has got out of sequence then this can be used to determine what that the first pattern was and that an error was detected. The error may have occurred before this (for example sending the wrong amount of data after a write command) and will only trigger when bit 7 of the rogue command is high.