We get loads of support calls along the lines of "I couldn't find user XYZ". It turns out they searched for a user to enrol them when they where already enrolled. Many courses have hundreds of students and you can forgive staff for not noticing the user already in the list.
It would be good if the user still turned up in the search but styled such that it was clear they were already enrolled.
This also applies to the 'assign role' screens.