Three ways this cartridge was wrong about itself

The run decided the verdict, not the machine

GB-CYC-07 shipped once in a form that zeroed the timer counter and then restarted the divider. Three cycles separate those two writes, and whether the tapped bit falls inside them depends on the counter phase the check was entered in — which is set by how much code ran above it.

So the verdict was decided by which checks failed earlier. It passed on one emulator and failed on SameBoy, and swapping the earlier failures swapped the answer.

Restart the counter first and zero the timer after it, and sweep more than one delay rather than sampling once. Anything that reads a timer edge and does not do both is measuring the rest of the run.

A subroutine that quietly ate the measurement

FlipSpeed runs the documented STOP-based speed-switch sequence and, like every subroutine in that file, makes no promise about which registers survive it. It loads A repeatedly on the way through.

A check that measured something into A, called FlipSpeed to return to the starting speed, then compared against A, was comparing FlipSpeed's own leftover value.

It does so silently, producing a plausible and constant wrong number rather than a crash. It shipped in GB-PPU-07's double-speed half and was caught only because the wrong number ($1) disagreed with an independent measurement on two unrelated emulators that had already established the right one ($2).

Two checks that skipped on every console, forever

This is the best of the three, because nothing about it looks wrong.

The build did not pass rgbfix -c, so the ROM's header byte at $0143 was $00 — and a real Color console boots a cartridge with no Color flag into DMG compatibility mode, where KEY1 and double speed do not exist at all.

Meanwhile the cartridge's own console detection still correctly reported CGB, because it reads the boot handover registers, which do not care about the cartridge's header.

So a double-speed check written the ordinary way built, linked, and skipped on every run, on every console, forever. Nothing looked wrong until somebody specifically asked whether it had ever run. Confirmed by building both ways and watching GB-CYC-09 and GB-PPU-07 go from permanently-skip to actually-run, with no change to any other check's verdict on any console.