Scaling too early
Engineering teams often focus on scaling a product before they have a product worth scaling, or even know if they'll need to scale it.
This is called "premature optimization", and it kills projects.
Focus on building something that delivers value first, and figuring out how to scale it second. If you don't nail step one, you will never get to step two.
This sounds simple, but it's actually very hard due to "performance shaming":
"Your app is nice, but you built it with...