A red-team run seeded eight small faults into ai-bookkeeper's estimatedtax.mjs (operator flips, off-by-one constants). The existing suite caught three. The five that escaped were a due-date comparison (>= on the asOf day), the zero floor and paid-amount term of balanceWithReturn, the wages-only effective-rate condition, and the Additional Medicare threshold shown in the warning text.
One new test pins each, and a rerun of all eight mutants now fails every one. No real defect turned up. These are tests of the arithmetic, not tax advice; estimates only.