Look-ahead bias

QuantConnect's glossary describes look-ahead bias as making a decision with information that would not be available until later, and says the bias invalidates a backtest because the test then uses data a live run would not have had. A moving average that includes a later bar, or a fill at the same close that was used to compute the signal, are two forms of it. Recomputing each decision from only the rows up to that date is one way to look for it. Finding or not finding it is not a statement about profit.