OP said "Mistake was getting caught" which to me implies that he is using the word "mistake" to reduce accountability rather than increase it. I think it's the latter: he's using the word "mistake" to take responsibility for having made a decision his users disagree with (whether or not it's objectively bad, I think is a moot point given the backlash from users).

Same clarification he makes in this tweet:

"I think you used "mistake" where you meant "accident". I never said it was accidental. We were treating it like a search query (which all big browsers do tag with an affiliate id to get paid from by the search provider). But a valid domain name is not a search query. Fixing."

https://twitter.com/BrendanEich/status/1269421487011713030?s...

It's the same thing as when FireFox adds it's referral code when going to Google Search, I believe that's most of their revenue...

TLDR: the bug was the referral code was being added in the autocomplete. It should have only been added to the exact affiliate address, such as "binance.us".