Sometime we all make mistakes. It can be pressing a wrong button, forgetting to close an order, opening a position bigger than intended, etc.
What should you do when you discover your mistake? The answer is plain and simple: close that position immediately regardless whether that accident benefited you or caused a loss.
As I said before, if there is no reason behind your trade, it should be closed. By reason I mean your trading system. So, if the trade does not fit your trading system, it should not be open.