The biggest mistakes developers make that leads to bad ending for many projects are
1. Huge Discounts
Many new projects are still making this mistake over and over again, huge discounts like 50% is insane and it always bother me how the hell the early buyers won't end up dumping since they have already see big profits.
2. Lack Of Solid Products and Ideas
Lack of working products and real use cases is why many investors are not taking new projects very seriously, no one would want to hold such tokens or coins for long, developers need to start working on how their project's token or coin will be very demanding throughout the globe
Personally, I think these are the real and main reasons and mistakes project developers and managers make that cause the downfall of most crypto projects. Giving investors a huge amount of tokens causes the potential of the token to lose its value on the crypto market leading to low volume and market cap. Developers most times do not really plan to develop products or projects that solve real problems in either the crypto ecosystem or the entire financial ecosystem.