On a typical floppy-based system, the act of reading a sector consists of the following steps:
Moving the head to the appropriate track.
Waiting until a certain "start of sector header" pattern of wiggles is observed passing under the drive head.
Reading a few bytes' worth of wiggles to see if they match the expected track and sector number, looping back to steps 1 or 2 if not (subject to some retry-count limits).
Waiting a little while until a "start of data" pattern of wiggles is observed, looping back to step 2 if it isn't.
Reading a sector's worth of wiggles.
Different systems used different patterns of wiggles for their "start of sector header" indication, and any portion of a disk which didn't contain the "start of sector header" pattern of wiggles expected by a drive would be effectively ignored.
Of particular note, if a disk format would contain 16 sectors per track, an attempt to read e.g. track 18 sector 1 would be processed in a manner oblivious to whether track 18 contained 16 sectors in a recognizable format, or contained a "sector 1" in the expected format and a bunch of other wiggles that did not include a "start of sector" header. It would thus be possible for a track to contain sectors formatted in multiple different ways, without the systems that are reading the track having to know or care about the various formats.
By my understanding, there were some programs released in Apple DOS 3.2 13-sector format whose first track contained a single sector written in 16-sector format, which in turn contained code to read a sector in 13-sector format, which in turn contained code to load everything else in 13-sector format. There are also many games that e.g. have a Commodore 64 version of the code on one side of a disk and an Apple II version on the reverse side. Are there any games or other programs which combine software for multiple independent machines using different disk formats, all on the same side of a disk, beyond the DOS 3.2 hack noted above?