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
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?
Indeed, many developers are repeating the same mistakes and some of them do that in purpose just to earn money. There are devs that create projects that are meant to fail thus, also failing the goal of cryptocurrency. This has been observed too many times in 2018 that caused the downfall of the reputation of the altcoins.