Look-ahead bias

Look-ahead bias is a backtesting error where a strategy uses information that would not have been available at the time of the trade, inflating results.

Examples include using a bar's close to act during that same bar, or referencing future data in an indicator. It makes a backtest look far better than reality and is a leading cause of strategies that fail live. Causal, bar-by-bar engines like Algovex's are designed to avoid it.