What it is
A Shadow System is any place where the real work happens outside the system of record: the spreadsheet that holds the authoritative number, the person who holds the actual process, the side channel where decisions get made.
The important reframe
Shadow Systems are evidence, not misbehavior. Each one marks the exact location where official infrastructure failed to do a necessary job, and someone competent filled the gap rather than let the work stop. Read them as a map of unmet requirements.
Why they're dangerous anyway
They're invisible to planning, unfunded, unversioned, and attached to specific people. They also quietly trade places with the official system — the dashboard becomes ceremonial while the spreadsheet becomes authoritative — and nobody announces the swap.
The rule
Don't eliminate the workaround until you understand what job the workaround is doing.
Where I've found it
Documented ≠ True · Every Company Has a Susan. Usually Several.