Vancouver Stock Exchange index truncation-error bug
Over about 22 months, the Vancouver Stock Exchange's official index drifted to roughly half its true value, a famous early example of accumulated computer arithmetic error.
The damage was public. The root cause was preventable.
Over about 22 months, the Vancouver Stock Exchange's official index drifted to roughly half its true value, a famous early example of accumulated computer arithmetic error.
What the software actually got wrong
The exchange's index-calculation software recalculated the index value after every single trade (thousands of times a day) but always truncated the result to three decimal places instead of rounding it, so tiny downward errors accumulated relentlessly until the index had to be recalculated from scratch and corrected.
The exchange's index-calculation software recalculated the index value after...
The exchange's index-calculation software recalculated the index value after every single trade (thousands of times a day) but always truncated the result to three decimal places instead of rounding it, so tiny downward errors accumulated relentlessly until the index had to be recalculated from scratch and corrected.
Requs AI Edge Case flags this exact pattern at the requirements and architecture stage — before a single line of code implementing it exists — so the assumption behind it gets challenged while it is still cheap to fix.
Beyond code coverage and "shall" testing
Root causes like this one rarely show up in code coverage or requirements-compliance testing, because nobody wrote a requirement anticipating the specific edge case that broke. Requs AI Edge Case and Requs AI Software FMEA are built to surface exactly this class of overlooked failure mode — before the software is written.
Surfaces this before code exists
Identifies edge cases like this one at requirements and architecture time, using the Common Defect Enumeration to catalog failure patterns seen across hundreds of real-world software failures — including this one.
Connects the failure mode to the hazard
Traces this class of root cause directly to the system-level hazard it can produce, so a defect pattern like this one gets flagged during design review instead of after it ships.
Find the overlooked root causes before they ship.
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