Poor choice of domain name.

Startups may make bigger mistakes. But that's the big mistake I most frequently see.

Cutting corners on the domain is not without cost. Frequently the company will pay very large sums of money to upgrade later. Why do they change their minds later? They leak traffic, lose money, and miss opportunities.

Frequently the cost to obtain the desired domain later on (after VC funding) is much higher than it would have been if the startup had locked down the domain early on.

And sometimes the optimal domain turns out to be unobtainable, forcing a startup to change horses in mid-stream with a rebrand that eats up time and marketing resources.

Really, it is often possible to secure an excellent domain without paying a fortune up front. Startups have no excuse, in my opinion. Just inattention or short-sighted haste.

Plan ahead, guys. Your brand name / domain isn't just a place holder.