You say Yusuf, I say Youssouf.
It's a wild world after all. It appears that Cat Stevens was detained due to a spelling error.
ADDENDUM by jodru
The larger issue is that we shouldn't be diverting planes because of a name on a list, for several reasons. First, quite clearly, the list is full of errors. The inaccuracy of the no-fly list has been established repeatedly through the detaining of people like Cat Stevens and Ted Kennedy.
Second, diverting an entire plane on the basis of one name match is a ruinous practice, much like the colour-coded Alert levels. Unnecessary diversions increase the boy-who-cried-wolf factor, thereby overstressing consumer patience and the resources of the airlines, not to mention security forces which waste time following false leads.
Lastly, a no-fly list is a no-fly list. The passenger shouldn't be on the plane to begin with. Homeland Security really needs to examine why it has to divert so many flights because people on the list have been allowed to board.