What was that D’oh moment as a new player?

Playing for a month and loving the game and the community. Admittedly I likely only know 5% of the game, but I just had a D’oh moment.

Running Power Armor. Not many Fusion Cores, so I treat them like gold. Well…been running around blasting bad things with a Tactical Gatling Lazer. Didn’t realize Fusion Cores was its ammo. I had to sit in silence for a while.

Curious about those moments when you first started that us tenderfoots can learn from.