Mostly write-ups of mistakes we made ourselves, since those are the ones we can show you the commit for.
An open source trading system asked FRED for the economy as it stood on 31 March 2023. FRED answered with the economy as it stands now, and nothing failed. What point-in-time look-ahead hands a backtest, measured, and the tells that give it away.
Read itA backtest reported 447.5% a year. It was wrong, and so were the two corrections after it. What several weeks of careful, validated, entirely wrong work looks like from the inside, and the protocol that came out of it.
Read it