[3] Examples can be found in MS-DOS/PC DOS: When MS-DOS/PC DOS 3.1 and higher (including Windows 9x) and OS/2 detect certain FAT OEM labels, they do not trust some BIOS Parameter Block (BPB) values and recalculate them from other disk geometry parameters in order to work around several off-by-one calculation errors caused by some of their formatter software under earlier issues of these systems.
[6][7] When MS-DOS/PC DOS 5.0 and higher are running on 286 or higher processors, the resident executable loader contains code specially designed to detect and fix certain widespread applications and stub loaders (such as programs linked with older versions of Microsoft's EXEPACK or Rational Systems' 386 DOS extenders) by patching the loaded program image before executing it.
[9] Over the course of development, DR-DOS also had to be modified to not only emulate many undocumented peculiarities and undesirable properties of MS-DOS and PC DOS (like having to use certain misleading filenames such as IBMBIO.COM, IBMDOS.COM or COUNTRY.SYS for files which do not follow the specs for executables under DOS,[10][11][12][nb 1] or having to introduce a directory path length-limited Current Directory Structure (CDS) internally[9][13][14]), but also actual bugs in the kernel and several drivers, in order to make certain other drivers and applications run on DR-DOS, when they were tested on specific versions of MS-DOS only.
[19] Another hardware example is found in the design of the IBM Personal Computer/AT A20 address line to emulate the behaviour in older processors.
[8][15] Microsoft Excel has always had a deliberate leap year bug, which falsely treats 29 February 1900 as an actual date, to ensure backward compatibility with Lotus 1-2-3.