Much better than the odds of terrorists mixing improbable liquid explosives in the lav of an aircraft.

I have all the ingredients for the concoction I described in my laundry room. Not because I'm a terrorist, but because I clean my home once in a while and don't like wearing dirty clothes. All the ingredients described are available in a corner drugstore, unlike the ingredients for the supposed liquid bomb threat. (in the concentrations required to make an explosive, peroxide for first-aid or bleach doesn't count as an explosive precursor)

Setting terrorists aside, IMO the odds of some ill-mannered high-school or fraternity pranksters pulling the ill-conceived stunt I described are pretty decent. All it would take is a couple of leaky containers being tossed in the bin. The ammonia/bleach reaction could happen by accident too; given some of the weird things some people bring to the checkpoint, who is to say ammonia and bleach might not be among them. But TSA merrily throws them all in the same bin.