Failing to learn one's social norms quickly enough, and failing to make any falsifiable tests as to whether I was making mistakes. I was nearly asocial in elementary school, middle school was just weird, and then high school was this horrible mess of thinking people were being freaked out by me, or avoiding me, or not avoiding me, or literally anything. In reality, lots of people loved me and I didn't need to fear or be awkward about asking favors of people or asking to hang out with them.

Buying big-ticket items such as computer equipment by numerical stats only. Compactness, physical construction quality, compatibility, and battery life (which is remarkably often not really rated, or degrades significantly) may be as important or more important than non-numerical quantities. For the specific example of laptop computers, this means to go for low-end Macs, business-level computers, and if you desire Linux, Lenovo Thinkpads. And the worst part is that I didn't end up spending that much less than I would have for something with much better construction quality, etc.

Not having any friends or interests outside of STEM (during university), or even outside a very specific nerdy mindset.