Seriously! It's been known for quite a while now, but according to US intelligence, wearing the Casio F-91W is a potential sign of someone being a terrorist. There's a whole list of Guantanomo Bay detainees who wore the watch, an old Washington Post article that said terrorists can use the watch to create time bombs and leaked Wikileaks documents linking the watch to al-Qaeda:
The possession of a Casio F-91W model watch and the silver-color version of this model, the A159W, is an indicator of al-Qaeda training in the manufacture of IEDs. The Casio was known to be given to the students at al-Qaeda bomb-making training courses in Afghanistan, at which the students received instruction in the preparation of timing devices using the watch.
Image credit: Wikipedia