Trading Halts and Limit Moves

A trading halt stops all trading in a stock or index for a set period, typically triggered by a large, fast price move (single-stock circuit breakers) or a market-wide decline crossing predefined thresholds (market-wide circuit breakers, e.g. the S&P 500 falling 7%, 13%, or 20% intraday). A limit move is a related but distinct mechanism common in futures markets: rather than halting entirely, the exchange caps how far the price can move from the prior settlement in a session, and trading can continue but only within that band — a contract can go "limit up" or "limit down" and simply stop moving further even though buyers or sellers still want to trade at more extreme prices.

Both mechanisms matter enormously for backtesting, because a simulated strategy that assumes it can always execute at the historical closing or next-bar price is implicitly assuming those prices were actually tradeable — which is false during a halt or a limit-locked session. A stop-loss that should have triggered during a single-stock halt might not have actually filled until trading resumed, often at a much worse price after the halt than before it; a futures strategy modeled to exit on a limit-down day may have been unable to trade at all that session.

Worked illustration: a backtest shows a strategy exiting a short futures position at the settlement price on a day the contract was "limit down" for the entire session. In reality, with no trading actually possible near that price, the exit likely would have happened one or more days later at a materially different, and probably worse, price — meaning the backtest's reported P&L on that trade is not achievable in practice.