1. Going all in from fiat
2. Taking a loan to invest
3. Not understanding the volatility of the market or the potential of what you are investing in, before investing.
With respect to the last point, you have to come to terms with the fact that the market is volatile and no one can give you a fact whether $369 would be the bottom for ETH at this moment or not, knowing the bearish condition of the market as a whole. The market could go lower if you end up buying, which is the reason you should always invest what you know won't make you have sleepless night and will enable you concentrate on the long term, since the essence is going long anyway.
Eventually, even if it would have to take some months or years, you will still be smiling at the end because this space is still evolving and I believe there is still so much to come in the long run. You may not be able to make trading decisions since you cannot trade, and you will be making mistakes following other people's idea, because you not having a strategy will make you lose out if the market turns the otherside, which makes trading not something to be toyed with, without having good knowledge and experience.
With all that being said, just stick with your guts, don't get greedy and focus on the long term, then you will be fine.