Interface and sandbox users
Currently, sandbox users are indistinguishable from normal users when visiting their user page or Special:Contributions (as can happen if one knows the username from outside the wiki); only the log will tell that they're not normal users. This is a bit confusing and "dangerous" because if one writes on their talk page they can't reply and may even be deleted in few hours.
It would be nice if the userpage, Special:Contributions etc. had a a notice/warning similar to what happens for blocked and non-existing users.