The end of the accounts

Today it's 2.25am instead of three a.m.... and after much labour I have traced the failure of *this* month's accounts to add up correctly to the fact that I entered the value of £10.85 of groceries including £2.91 of ox-tail (already entered separately) as 10.85+2.91 instead of 10.85-2.91.

Just how many different mistakes is it possible to come up with when trying to correlate two sets of figures recorded in two different ways? :-(