That second when you realise that you really can't just wing it, and you actually have to calculate something, or take it more seriously.
Buying a few rolls of insulation for the loft, then having the moment of "wait, this isn't just two rolls worth, I have to actually add this up, and order for delivery"
Loading up the car for a trip to the dump, then noticing that it's riding a bit low, and finding out that 9 bags of rubble is actually 50% of the total kerbweight.
In my world, the day before the grass arrived I would suddenly discover that I needed to do another 3 days of work preparing the ground. Then I'd be stuck with rolls of it sat in the living room for a month!
I took my drone to go capture some footage of the flying Scotsman. It took ages leaving the station and I was on 10% as it crossed the bridge. Well I made no calculations at all and took a gamble.
Long story short I ended up climbing a large tree not much later.