> a long-standing bug was accidentally fixed

In some cases (e.g. in our case) long standing bugs become part of the API that customers rely on.

It's nearly guaranteed, even if it is just because customers had to work around the bug in such a way that their flow now breaks when the bug is gone.

Obligatory: https://xkcd.com/1172/

	▲	thaumasiotes 4 days ago \| parent \| next [-]
		That comic doesn't show someone working around a bug in such a way that their flow breaks when the bug is gone. It shows them incorporating a bug into their workflow for purposes of causing the bug to occur. It isn't actually possible for fixing a bug to break a workaround. The point of a workaround is that you're not doing the thing that's broken; when that thing is fixed, your flow won't be affected, because you weren't doing it anyway.
	▲	giaour 5 days ago \| parent \| prev [-]
		Also known as Hyrum's Law (https://www.hyrumslaw.com/), but more people know the XKCD at this point :)