No idea if this is intentional, but it actually might be a trap for few people:
Talking about create_user function (Simpleauth), which may lead to unexpected behaviour, as the username check is case-sensitive, in opposite to the email address.
Fuel makes a direct check in the query, not making any case-checks:
->where('username', '=', $username)
(Yes, I know MySQL queries are case insensitive, but it depends on the MySQL binary configuration)
This can lead to having multiple users with the same name (admin and Admin). If this intended, should be configureable in my opinion, because it might fool some people wanting unique names.
Btw: looking at the function, makes me think of another tiny bug: you map all returned fields to lower and check if email is there, so if the $username is an used email address (unlikely, but can happend in certain circumstances) it will throw an invalid exception saying that email address is already in use - and user email may not really be used by another user.