The biggest mistake some people make is focusing on the current price of bitcoin, whether it’s cheap or expensive isn’t really a criteria to know when to buy or not. No one has the slightest idea of what the price might look like in the coming years, so waiting for the perfect entry can become another form of procrastination. The most important thing should be the entry price but having a realistic amount to invest with, having a reasonable amount for your emergency or for backup and having and maintain a long term mindset to hold your asset through thick and thin times, especially during moments that the market doesn’t move in your favor.
Another error most investors still do is putting money that they might need in the near future. Sometimes your original plan is to hold for long term but when unexpected events occur you'll have no choice than force yourself to sell at a bad time, as you didn't give yourself enough financial breathing space.
I also believe that sometimes people will start increasing the investment amount just because they see the market price going up. When everyone is excited, it becomes easy to get tempted to invest more than you can afford, but excitement is not enough reason to go above what you can comfortably afford. To me, it's better to have a budget and stick to it than to follow whatever is going on in the market.