I came across this question about visual acuity on Quora and eagerly answered it.
"Oh, this is just converting to a Snellen equivalent," I thought and happily described the math needed to convert an acuity value of 6/9 to one of 20/whatever. I did the math and derived an answer. Voila! I am awesome at Quora! I so rock!
I decided to check my answer at this site, which confirmed my math. But it also showed me that vision tests could be done in meters. I know, what a concept, right?
So I added a qualifying paragraph into my answer, stating that I was presuming that 6/9 was measured in feet, but if it referred to meters, then some adjustments to calculations might need to be made.
Only then did it slowly dawn on me that, hey, most of the planet uses the metric system. In fact, only the United States, Myanmar, and Liberia still use the imperial system, and Myanmar plans to switch to metric soon, if it hasn't already. So this question about a 6/9 acuity probably didn't need a Snellen equivalent at all; the asker probably just wanted an explanation of how visual acuity measurements are derived and what they mean.
Egg on my face! I sounded like a blithe, US-centric jerk who didn't even notice the eye-rolls!
So, again, I rewrote my answer and exchanged the presumptuous pronoun 'we' for 'I' in a couple of places where it desperately needed to be changed. I added some commentary about being American to explain some of my presumptions and then gave the answer that was probably desired in the first place.
It's scary to realize how easy it is to write as if your culture is the be-all and end-all of the world. It's scary to realize how easy it is to presume that of course, everyone thinks as you do--and all of this with no intention whatsoever of offending anyone.