From above you can understannd that Trial Balance is not tallhing because this is a one-sided error. Similarly if Sales Book is overcast by Rs.1000 Trial Balance will not tally because Debit side of Trial Balance (representing debtors) will be short by Rs.1000 because credit side of Trial Balance (represented by Sales A/c) is overcast by Rs.1000/
If purchases are recorded as Rs.10,000 instead of Rs.1,000, trial balance will tally because it is a two-sided error because from journal Purchase a/c will be debited by Rs.10,000 and Creditor's A/c will also be credited by Rs.10,000.
In case purchases are by mistake are recorded in Sales Book, Trial Balance will tally because on the one hand Sales A/c will be credited and on the other parties A/c will be debited. It is a two sided error, hence Trial Balance will tally.