I guess I'm a bit of a nitpicker compared to others.
I keep every ATM, EFTPOS and CC receipt and every couple of days I update a spreadsheet with my purchase, withdrawals and deposits, one sheet for each account. That way I can immediately check the balance of any account even when the net is down. I receive statements electronically and check them against my spreadsheet items. I have only picked up a few discrepancies over the years.
For mortgages on my investment properties, I actually calculate the interest and compare with the banks calculation. I have found a few bank errors, but nothing that would indicate they were intentional.
At the end of each year, I archive all my spreadsheet data for that year and keep the archive for 5 years as I do with my income tax data.
Apart from keeping on top of errors in my statements, the spreadsheet data can be useful for searching on past expenses without needing to keep paper records. For example, when I want to compare last year's water rates with today's. It also makes doing my tax return a lot easier.