Graduation
Graduating with a B.Sc. in Nuclear Engineering — the analytical foundation everything after was built on.
Four years of thermal-hydraulics, control systems, and probabilistic risk assessment don't obviously lead to a software engineering career, and at the time I didn't know they would.
What I didn't appreciate until much later: reasoning about failure modes in a reactor system and reasoning about failure modes in a distributed system are the same muscle. Both punish guessing and reward modeling the system before trusting any result it produces.
I went on to start an M.Sc. in the same field, but this was the day the foundation was set — the habit of isolating what actually matters before touching a solution, which now shows up in every architecture decision I make.