At work today a colleague saw me on ux.stackexchange and we got talking about it. Turns out they used to be a semi-regular user a while back, but can't remember their login details. They do remember their username, but not the email address they used (it was some time ago and they've had many email addresses over the years). Not only do they not remember which email address they used, but it's likely to be one they no longer use.
So, how do they recover their account? The recovery page only asks for an email address and provides no other options.