I had an IRS problem once connected with the sale of some rental property.
It all boiled down to my holding a sum of money in an account for ten days, when I was only supposed to for 7 days. The fact that the 10 days contained Christmas and New Years holidays was not taken into account.
If I had been able to move it after 7 days I would not have had to pay taxes on it, but since it took ten days to move it I owed the full amount.
Nuts.