The Accounts team just produced a Board report on our financial situation. An
email was sent out with an excel sheet attached. Both I and a colleague
opened the file direct from Outlook and both saw significantly different
results reported.

His version of Excel had switched the Calculation tab (Tools/Options menu)
to Manual - mine however remained as Automatic and reported the correct
numbers. Despite having the same formulas in the same cells his was not
summing the equations correctly. Why is this?