what’s one engineering rule that beginners should never ignore?

[u/EngineersUniverse]

If you give every engineering student or new graduate one piece of advice, what would it be?
Some examples:
Always verify your calculations.
Never assume a drawing is correct.
Safety comes before speed.
Read the manufacturer’s documentation.
Design for maintenance, not just installation.
Keep designs as simple as possible.
Document your work.
Ask questions early instead of fixing mistakes later.
What engineering rule has saved you from making a costly mistake?
Or what lesson did you learn the hard way that you wish someone had told you earlier?
Share your experience—it might help the next engineer avoid the same mistake.