Confession is good for the soul, and too many tech people are incapable of admitting they made a mistake. So with that in mind:
I wasted several minutes of someone’s time this week because I mistakenly thought a URL was syntactically invalid. It was invalid according to RFC 1738, but I’d forgotten the EBNF for relative URLs in RFC 1808.
Apologies to anyone who thought I was infallible.