Overfitting is real, but what's the actual cost?

Seeing a lot of chatter about overfitting lately, and it's definitely a topic that hits home. It's easy to get caught up in curve-fitting a backtest to look perfect, but then it falls apart in live trading. I'm really curious to hear how others quantify the downside of this. Is it just missed profit, or are there other hidden costs like increased drawdown or unexpected slippage that we should be tracking?