Even the most experienced traders sometimes make inadvertent mistakes. For example, they might enter a trade by accidentally clicking their mouse, or they might enter a trade using the wrong type of order (e.g. a limit order instead of a stop limit order). Making such a mistake is not a problem as long as the reaction to the mistake is prompt and correct.
Exit a Mistake Immediately
The correct way to react to an accidental trade is to exit the trade immediately. The trade should be exited using a market order, because market orders are always filled regardless of any intermediate price movement. The temptation is to stay in the trade to see if it becomes profitable, but this reaction usually results in a losing trade.
What Counts as a Mistake
A mistake is any trade that does not meet your trading system's requirements, irrespective of the reason that caused the mistake. It doesn't matter if you misread a chart, placed a limit order instead of a stop limit order, or if your cat walked across your keyboard. If a trade has been entered when there is no sign of any trading signal, then the trade is a mistake, and it should be exited without any hesitation.
However, entering a valid trade, but not receiving the exact price that you wanted, does not count as a mistake. If all of your trading system's requirements have been met, but your entry order is filled one or two ticks away from your preferred price, the trade should be held. The correct response in this case is to adjust your target and stop loss accordingly, but not to exit and re-enter the trade.