What would cause a user's activation link to fail? I've just rolled out some changes to a site that's now seeing a big increase in (front end) user sign ups.

I have activation by email on and I'm tracking in analytics the number of hits to the activation success and activation failed pages. 

If I look at yesterday as an example, 168 Success and 15 Failed. 

I'd like to start working on bringing this down, but it would be useful to know the scenarios that would cause a user to hit the failed page. I believe they'll get this if they, for example, click the activation email twice. 

Is there any other obvious causes that I use as a starting point?

Thanks!
They could already be active Users
Their activation link could have timed out


1) happens more often than I'd think, people try using it as a way of "getting to the site again"

2) Can be controlled in /config/general.php - https://craftcms.com/docs/config-settings#verificationCodeDuration