No Overpromising
Early in my career I said yes to a client timeline I privately knew was unrealistic. I thought I could make it work by pushing harder. I couldn't. We missed the date, the client was blindsided, and I spent three weeks recovering a relationship that had been fine until I tried to manage it instead of being honest about it.
I never do that anymore. If a timeline is too tight, I say so in the first conversation — with the numbers to explain why. If a feature is riskier than the brief implies, I flag it before scoping, not after. If I genuinely don't know how long something will take, I say "I don't know yet — here's how I'll find out."
Some clients don't like hearing this. That's fine. The alternative — telling people what they want to hear and then underdelivering — is worse for everyone, and it's not a business I want to run.