Or more commonly:
DO NOT WRITE, STAMP, OR SIGN BELOW THIS LINE
RESERVED FOR FINANCIAL INSTITUTION/BANK USE
This rather intimidating warning appears under the endorsement signature space on most checks, often accompanied by a dim gray separating line. Obviously, the banks are quite territorial about their paper-space and don't want just any old ne'er-do-well muckity-mucking up their pristine pulpy sheets of monetary exchange--at least, no more than is absolutely necessary.
A sparsely supported theory is that the financial institution reserves this space so that they might use it for internal processing of the check. They supposedly place meaningful memorandum and tracking data in this restricted space, and that this application--for whatever reason--makes the check processing system more dynamic and less likely to be hindered by errors; however, the general consensus is that this idea is bunk.
NOTE: The author would like to strongly caution those who would be so bold as to defy the boundaries and mark below the line: Checks bearing these rebellious marks have in some cases been subjected to a torturous process known as 'voiding', in which the said check is marked in such a way that all other banks shun the check. This is the most effective means of ostracizing a check, it is the equivalent of A Scarlet Letter for the document world. In the unfortunate event that one comes into possession of a 'voided' check, he should be advised not to panic. The best thing to do with a 'void' check is to store it in a cool, dry place, where you can easily find and recover it if the need so arises.