Passage of a new tax law, the use of significantly different tax tables and computer errors by banks and other institutions are producing some of the most common errors in tax returns, according to IRS.

This year, the IRS changed the tables taxpayers can use to determine their liability, and it has resulted in some confusion.

Last year, taxpayers did not have to subtract personal exemptions from adjusted gross income when using the tables to determine tax liability.Instead, the taxpayer would go to the table for persons with their number of exemptions and income.

This year, however, the taxpayer must subtract personal exemptions from their adjusted gross income. Those who do not will end up paying too much tax.

Tax specialists said they have begun to receive complaints from persons who do not understand changes in the tables. These people say they appear to have gotten a tax increase, despite the 1981 tax cut.

In addition, instead of four separate tables for single taxpayers, married filing jointly, married filing separately and head of household, these categories have been consolidated. Persons with income up to $50,000 will be able to use the tables.

Another problem that has emerged involves the exclusion for interest and dividend income. Singles can exclude up to $200 an a joint return can exclude $400.

This year, the IRS changed the forms used by banks and other financial institutions to tell their customers what portion of their interest income can be excluded. In some cases, however, the banks did not change their computer programs and the result is that a number of taxpayers have received forms that fail to disclose the fact that a certain portion of their interest income can be excluded.

Among the other common mistakes that have been noted repeatedly in recent years are:

* This failure of working poor people to claim the earned income tax credit. For those making less than $10,000, there is the possibility of getting a tax payment from the government, but many persons do not realize they are eligible.

* Persons claiming medical deductions often do not realize that only the amount in excess of 3 percent of their income can be deducted. Failure to make this calculation results in taking too large a deduction.

* Similarly, persons deducting theft and casualty losses often deduct the entire amount without subtracting, as required by law, the first $100.

* Persons who worked part of the year and collected unemployment benefits for the rest often miscalculate their tax liability. If their combined income from work and unemployment benefits is below $20,000 for a single person or $25,000 for a joint return, none of the unemployment benefits is taxable. If, however, they exceed these amounts, one half of the unemployment benefits is taxable in so far as they push total income above these ceilings.

The other most common mistakes involve basic adding and subtracting.