IBM trainee programmer reduces 4-byte program to 2
Some time around MVT 18 on IBM mainframes, a trainee IBM programmer in the new version release team had a brainwave, and thought that he could reduce the size of the 4-byte IEFBR14 program used in JCL as a 'don't-do-very-much program' to just 2 bytes, so as to save 50% of the space it required, and have it run faster, too!
IEFBR14 consists of
SR R15,R15 ; set the return code register to zero
BR R14 ; branch to the return address in register 14
He removed the first instruction, so the contents of R15 were undefined, and could have any value. This caused considerable unhappiness to occur with all jobs.
It is not known whether the programmer's employment with IBM was continued...