Look-Ahead Bias
A flaw in backtesting where a calculation uses information that was not yet available at the moment it claims to act, producing results that could never have been achieved live.
It is the easiest error to make and the hardest to see, because the output looks like a good strategy rather than like a bug. It can be as subtle as measuring a two-signal setup from the earlier signal's timestamp when the later one had not yet fired.