As per an article in the May/June SPMC magazine Paper Money, written by Joe Farrenkopf;
Certain sheets in every production run are known internally at the BEP as "reader sheets" and are regularly pulled out of production for inspection. A reader sheet may be returned to the stack , or more commonly it will be replaced with a sheet from a previously serialed sheet-replacement star run. Reader sheets that routinely replaced with star sheets include the first two sheets from every stack of 10,000 half-sheets (i.e., those sheets whose serials end with 0000 and 9999, often referred to as "rollover" sheets by collectors), as well as the second-to-last half-sheets of the completed production run (i.e., sheets with serial numbers 00000002, 03200002, 06400002, etc.) The BEP implemented the routine replacement of the 0000/9999 reader sheets in order to reduce the number of defective notes that were periodically created at the start of the overprinting of each stack of 10,000 half-sheets. That routine replacement is the reason that notes whose serials end in 0000 and 9999 are generally not found in circulation.
I found and bought a GEM pair of these many years back from a seller on
ebay. His name is Harvey, and his
ebay store was known as Harvey's Money Store. He specialized mostly in modern star notes and fancy serial numbers.

Now that the BEP has switched to LEPE and 50-subject sheets, this process has been discontinued and 0000/9999 notes are more readily found in circulation.