A “thought bug” is a bug in a bit of software or a website etc that is caused not by bad programming but by designers not thinking.
Here is an example of a thought bug.
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Take the person you most commonly email and find an email from them and click the "Report Spam" button. They will now be sent into the spam folder, as will future mails from them. That’s the thought bug. Gmail should say "You have emailed that person 8,345 times, are you sure you want them to be CONSIDERED SPAM now?"