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 seen big profits.
It is their marketing and they are allowed to do it, i don't see anything wrong on this but they must LOCK the token of those who bought at a discount price like pre-sale clients.
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
Lack of idea can be counted as in cryptocurrency, idea is the best thing to add in your project more unique and useful for the future is the one who stands up. but the working product? i don't think so it is better to have out one the token is already in exchange to give a boost on the price!
3. Bad Exchanges
Imagine new projects that make over millions of dollars in ico or ieo and end up listing on bad exchanges with no enough volume, what do they expect to happen?
If they can't afford the best exchanges at least go to the average one and not to the market with no value or popularity! and then they can be on the best at later time once ready!