"errorfree" simply means "exact baud rate match".
A small percentage error is acceptable (lots of people use the "error-prone" rate of 9600 on the 16MHz Arduino without problem), because the receiver is resynchronised every character.
Only if you had really long word lengths would cumulative timing errors become a problem.
Rate mismatches are much less of a problem with crystal-controlled receivers and transmitters at quite high rates, compared to the bad old days of mechanically-timed slow rates from teletypes and telex machines.