Some traders often invest when they say that the market is bullish which I think is wrong, they need to analyze patterns, they need to learn candlestick and some other strategies that can help them to fully understand the market and let them not suffer to much loss. Because sometimes we can encounter fake pump as well, so we need to be extra careful in these things.
If someone starts investing following the DCA method. Then it should be ok to invent even in the bull run. When someone hears that the market is up now and will up further, and they want to take advantage of it; they are newbies to the market. You always have to invest when the market is low, and you can buy more satoshis at a low price.
But, if someone wants to hold it for a long time and keep accumulating satoshis then they might buy Bitcoin in the bull run too. Unfortunately, this is the common mistake that newcomers do. They hear about Bitcoin once it start to going up and they think it will pump further and they decide to buy Bitcoin at a high price.