common spreadsheet errors and consequences

Using the internet, search for information regarding common spreadsheet errors and consequences.  Choose at least two types of errors.  Your post should include a discussion of the error, an example of the consequence(s), and proposed a method(s) for prevention.

Examples of typical Spreadsheet errors:
·         Improper or incorrect formulas used
·         Data entered or copied incorrectly
·         New data posted, Excel model did not update correctly