Things broken on purpose

Caches solve a problem and create three new ones: invalidation, observability, and the slow drift between what the cache thinks is true and what the source thinks is true.

I keep a small log of decisions in a text file. Not architectural — just small ones. "Picked X over Y because Z." Future me reads it more than I thought he would.

If a config file has more than a dozen options, half of them aren't being used. The other half are being used in ways the original author didn't predict.

There's a kind of premature optimization that comes from imagining a future team. "What if someone needs to swap this out?" Often, no one does.

Latency budgets work better than latency targets. Targets are aspirational. Budgets force you to delete features when you go over.