bug door

n. [common] A hole in the security of a system deliberately left in place by
designers or maintainers, implemented in the form of a deliberate but subtle
and thus hard to spot bug. This can be implemented as a slightly mis-spelled
variable, a typo, extra or missing parentheses or a confusing regex. If the
bug door is subsequently found then it can be plausibly denied as a simple
programming mistake.

Also see back door.