The first one is sticking to trading with only one currency. Traders often prefer their country's currency in the positions they open. This is because there is no time zone difference and other factors such as news or data can be followed easily. On the other hand, if instruments in different time zones are preferred, the data released during the day cannot be followed easily, and you may face a big surprise when you wake up in the morning. However, this may limit your scope of trading, thus try to avoid trading based on only one currency and expand your portfolio to become more active in the market.
The second most common mistake could be the lack of attention to collateral. Traders should consider their collateral before opening a position. Because if you open a position on an instrument, which has high volatility, with a high lot size, a tiny movement against your will may end up as a 'margin call', so try to learn the average volatility of an instrument before opening a position.
Similar to sticking to trading with only one currency, many traders stick to one instrument. They trade only one instrument, or most of their trades only include one instrument. On one aspect, this might be considered a ‘true strategy’ by some experts because it minimizes your risk compared to trading many instruments. However, this move increases your opportunity cost. You will miss opportunities to gain profit on other instruments. In addition, trading multiple instruments can eliminate the risk of losing by hedging another instrument. For example, if you open a position in the EURUSD currency pair and suffer from losses, you can open a position in Gold or Silver and minimize your risk. Last but not least, many traders are not good at arranging their time. Managing your time wisely is a key to success and the forex world. And you should also follow data releases and take action accordingly.