Sign errors and upside-down things

When was the last time you made a sign error?
When was the last time you visualized something upside-down by mistake?
I thought so.

Marcello Herreshoff writes at Overcoming Bias on using the native architecture, i. e. taking advantage of the things that the human brain is naturally optimized for when trying to do more recently invented things, like mathematics.

(Although I did visualize something upside-down today. In particular, I drew the Hasse diagram of a certain poset and then realized that the way I was thinking of things, I really wanted the reverse order. But that doesn't really count, because I'd never seen this particular poset before.)