Algo trading backtest shenanigans part 1

Algorithmic science is one of the world's most accurate and fact-based subjects. Yet, In algo-trading/quant trading, one of the biggest lies you will encounter when presented with a model's theoretical performance is front and center and called -- backtesting. Backtesting, at least in theory, should be the assessment and evaluation of a given model's theoretical performance. "If I would have traded stocks based on the predictions of my model from year X up to year Y, then I would have had a yearly return of Z percent. So- trading based on the current predictions of the model, I should get the same yearly return."