Harry Potter actress Emma Watson used to model for Burberry. The luxury-goods retailer could do with some magic now.

Its shares slumped 21% Tuesday after a trading update signalled same-store sales were flat in the last 10 weeks against a year ago, and started to fall toward the end of that period. For investors, it is a sign that not even Burberry's famous trench coats can protect it from ill winds affecting the luxury sector, and that Europe isn't the only part of the global economy that is falling out of fashion.

Certainly, slowing Asian growth