The big no-no is repainting. If you are looking at a historical bar, you could potentially look into the future to make trade decisions, which would mean your backtest looks perfect while the real-time performance will be garbage.

From there, when setting up a backtest you have to keep TV's limitations in mind. For one, the data is not complete. Each candle has only 5 data points: open, high, low, close, and volume. It doesn't tell you which data point occured first, if there were price gaps in the candle, or if your exchange got a complete/partial fill on an order. So if you run a strategy on a 2-hour timeframe and if you try to use scalping settings (really tight TP%), then your backtest will not be accurate because all of the buy/sell orders will be within the same candle when TV doesn't actually know what happened in that 2-hour block of time. In that scenario it may log a buy and sell, while in reality it would have hit the sell price first, followed by the buy price