What goes wrong
Early assignment on a short leg, a dividend that makes exercise rational before expiry, pin risk at the strike on expiry day, and a broker auto-liquidating a position whose margin requirement changed overnight. None of these appear in a backtest and all of them appear in a live account.
Why this resists automation
Handling assignment requires knowing what the resulting position is, whether it breaches margin, and what to do about it before the market opens. That logic is bespoke to the broker's rules, and broker rules are documented inconsistently and change without notice.
The practical response
Run every structure on paper through at least one full expiry cycle including an in-the-money expiry before committing capital. The purpose is not to validate the edge — it is to discover which parts of the plumbing you have not built yet.