The form author can enter error checking formulas that automatically check for mistakes as you fill out a form. When you enter a value and then tab out of a cell, it is checked according to the field's check formula. For example, a check formula might check to ensure that a field's value is within a particular range. If an error is found, a message box appears when you try to click or tab to a different field.
If you enter the wrong type of information in a field (e.g., if you enter text in a field that requires numbers, an error message may appear). Depending on how the form author has configured the form, you may not be able to tab out of the field until you have entered the correct value. In addition, the form author may prevent you from printing or saving a form with invalid data. In this case, when you try to print or save a form, a message displays to inform you that required information is missing from your form.
If you enter a value in a lookup cell, and then click a button on the toolbar without tabbing to or clicking a different cell and a lookup error occurs, the toolbar action will still be performed. If you click more than one toolbar button, the actions take place in the order that the buttons were clicked.
For calculations and data lookups that take a long time to process, if an error is detected and you continue to type information in the other cells on the form, the information will be erased or rolled back to the previous value. The cursor is automatically returned to the cell with the error. You must correct the error before you can continue to enter data in other cells. This ensures that the error is corrected if you're typing fast and do not notice the error.