During the current month, the following errors occurred in recording transactions in the purchases journal or in posting from it:

An invoice for $1,875 of supplies from Kelly Co. was recorded as having been received from Kelley Co., another supplier.

A credit of $420 to Blackstone Company was posted as $240 in the subsidiary ledger.

An invoice for equipment of $4,800 was recorded as $4,000.

The Accounts Payable column of the purchases journal was overstated by $3,600.

How will each error come to the bookkeeper's attention, other than by chance discovery?