When a formula in a table cell is incomplete, contains invalid cell references or is otherwise incorrect, or when an import operation creates an error condition in a cell, an icon is displayed in the cell. A blue triangle in the upper left of a cell indicates one or more warnings. A red triangle 
in the middle of a cell means that a formula error occurred.
To review the error or warning, tap or click the icon.
A message window summarises each error and warning condition associated with the cell.
Check for typos in your cell references and values.
Make sure the formula doesn’t include a reference to itself — for example, if the cell is part of a range.
If you want to include text in your formula, put it in double quotations. See Tips for using double quotations in formulas.
Make sure that number values you typed or pasted into cells are set to a numeric cell format in the inspector.
If you delete a referenced cell, update your formula to no longer include it.
Check your formula after you insert rows or columns to ensure your references still include the cells you meant to include.