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Sometimes it happens when it takes a bit longer to answer a question (got interrupted or something), that we get a captcha. Why is this?

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Since the page shown is a hip reference to a FOTC video about robots, I've always imagined that there was (or still is) a robot in the wild that deliberately takes a long time between a form fetch and a posted answer.

It must be thinking that returning a form with 1K characters in a field only 100 ms after fetching the page would obviously not happen with a human, so it should wait for a while before answering.

So it waits. And since we get the "I'm a human" page after several (ten?) minutes, it must wait quite a while.

Unfortunately, the threshold time that causes the CAPTCHA page seems to be just about as long as it takes to actually verify the facts of an answer as well as get distracted by at least one unrelated Wikipedia link and possibly one phone call from a customer wondering if you've finished his project (that he hasn't authorized to start) yet.

Edit: I've expanded and tried to clarify the point I wasn't making as well as I could have.

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  • But Ikke's point is that it's doing the reverse - it's showing the CAPTCHA if he does take a long time.
    – Jon Skeet
    Jul 3, 2009 at 7:49
  • Right. I've said it badly, I think. Editing...
    – RBerteig
    Jul 3, 2009 at 8:14
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To trigger CAPTCHA:

  • The minimum window is 5 seconds
  • The maximum window is 40 minutes

basically we are flagging anything that looks suspicious, and forcing users to prove they're not bots.

edit: due to popular demand, I removed the maximum time check -- so only minimum times are checked when posting now.

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    Why is there a maximum window? To discourage outdated answers?
    – sth
    Jul 3, 2009 at 23:28
  • I seem to hit that 5 second minimum frequently when fixing typos. I had always thought that it was something about the frequency of submitting changes, rather than simply the time before hitting submit. Now that I know what it is, I'll take a breather if I'm working quickly!
    – Greg Hewgill
    Jul 4, 2009 at 1:14
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    If a user solved one Captcha for 40 minutes couldn't the limited grow the next time by 50%?
    – Christian
    Aug 11, 2009 at 22:05
  • Have you done any real testing on this? How do you know that a post is "suspicious" because it takes more than 40 minutes? I frequently hit this limit (it's what led me to this question.) If bots store cookies indefinitely you can probably increase that (to 2 hours or more) without significantly increasing the spam.
    – Perpetual Motion Goat
    Feb 3, 2010 at 22:15
  • The crowds are cheering +1. Thanks!
    – alex
    Feb 4, 2010 at 9:27
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Could it be possible that the session cookie has expired, so it looks to the server that you're jumping in and answering as the first action in a new session. I'd class posts without a session cookie as suspicious (on the premise "that you've come straight to posting without reading the form")

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