Does this common math misunderstanding have a name?

I'm not quite sure what you're asking, but one way that fractions (and by extension, ratios) are tricky is that a larger denominator gives a smaller value. Five is greater than three but a fifth is smaller then a third. I've seen my children struggling with this.

"It's an error so common that I imagine it must be named by now."

Not really an error so much as an attempt to mislead.

"While we're at it, is there a name for when people tell you meaningless things like, "I doubled my sales", only to discover the sales went from 1 unit to 2. It's an error so common that I imagine it must be named by now."

Don't know if it has a word but it has its own xkcd comic.

https://xkcd.com/1102/

This list is rather daunting to check for a match against this example: https://en.wikipedia.org/wiki/List_of_cognitive_biases It's either in this list, or we should describe this bias as new?

If we don't discover a name, I guess we could call the first one the "Small discount, large increase equivalence?" sounds pretty bad. Anything with the word numerator or denominator in it would be far worse.

On the second one, "unqualified percentages?"

A good example of "I doubled my sales" would be "My risk of getting cnacer has doubled" - this might mean my risk has gone from 1 in 100,000 people to 2 in 100,000 people.

I found the answer. It is called the Framing effect. https://en.wikipedia.org/wiki/Framing_effect_(psychology)

I would posit that there could be benefit in naming the more specific form of discount/increase, as its hard to derive benefit from the knowledge of a general "framing effect" as framing is more often non numerical.