Copper coins is counting a gold when you check you balance at the bank. I thought I was going crazy trying to make checks.... but started looking at all the coins I have and that was the culprit.
Ya, this was reported to Telamon early in the process. The "balance" has actually always given you the total of all coin piles in your bank, Gold, Hcoin, and Plat. However, copper tends to pile up in amounts large enough to make that balance look incorrect. @Chris could comment but last I checked he said its not an easy fix to only call out amounts of unique hue items vs. items (coins) in general.
This has caught me out a few times too. Need a small 'moneybox' in the top corner of the bank box that only holds coins and isn't counted by the banker.
totally different note: a cool idea would be that balance would actually also count the checks in bank... accumulated with the "coins" or a new command that counts only the check values.